MCQs: Amino Acid Catabolism, Inborn Errors, Amino Acid-Derived Neurotransmitters & Heme

Catabolism

Quick Revision Notes: Amino Acid Catabolism, Inborn Errors, Neurotransmitters & Heme Catabolism

1. Catabolism of Phenylalanine and Tyrosine

Phenylalanine → Tyrosine

Phenylalanine is an essential amino acid.

Reaction:

Phenylalanine
↓ Phenylalanine hydroxylase + BH₄
Tyrosine

• BH₄ (tetrahydrobiopterin) is required as a cofactor.

• Tyrosine is therefore considered conditionally essential in PKU.

Catabolism of Tyrosine

Tyrosine
↓
p-Hydroxyphenylpyruvate
↓
Homogentisate
↓
Maleylacetoacetate
↓
Fumarylacetoacetate
↓
Fumarate + Acetoacetate

Easy memory

Tyrosine gives:

Fumarate + Acetoacetate = Glucogenic + Ketogenic

• Fumarate → glucogenic

• Acetoacetate → ketogenic

2. Important Disorders of Phenylalanine and Tyrosine Metabolism

A. Phenylketonuria (PKU)

Cause

• Deficiency of phenylalanine hydroxylase.

• Less commonly, a defect in BH₄ metabolism.

Biochemical change

Phenylalanine ↑↑
↓
Excess converted to phenylketones

Important features

• Intellectual disability if untreated

• Seizures may occur

• Musty/mousy odor of urine

• Reduced pigmentation may occur

• Neurological damage due to disturbed brain amino acid and neurotransmitter metabolism

Treatment

• Restrict phenylalanine

• Give tyrosine because it becomes conditionally essential

• BH₄ supplementation may help selected patients with BH₄-responsive disease

Exam point

PKU = Phenylalanine hydroxylase deficiency

Mnemonic:
"PKU = Phenylalanine Keeps Up" → Phenylalanine accumulates.

B. Albinism

Cause

• Defect in melanin synthesis

• Classically due to deficiency of tyrosinase

Pathway

Tyrosine
↓ Tyrosinase
DOPA
↓
Melanin

Features

• Decreased or absent melanin pigmentation

• Very light skin and hair

• Reduced pigmentation of eyes

• Visual problems may occur

Exam point

Tyrosinase deficiency → Albinism

C. Alkaptonuria

Cause

• Deficiency of homogentisate 1,2-dioxygenase

Result

• Accumulation of homogentisic acid

Important features

• Urine becomes dark/black on standing

• Ochronosis: bluish-black pigmentation of connective tissues

• Arthritis may develop

Exam point

Alkaptonuria = Homogentisate oxidase deficiency

Memory trick:
"Alkaptonuria → Alkaline-looking dark urine"
(Think of urine becoming dark after exposure to air.)

D. Tyrosinemia

Important types

Tyrosinemia Type I

• Enzyme deficiency: Fumarylacetoacetate hydrolase

• Toxic metabolite: Succinylacetone

• Major organs affected:

  • Liver

  • Kidney

• Increased risk of liver failure and hepatocellular carcinoma

Treatment:

• Nitisinone

• Dietary restriction of phenylalanine and tyrosine

Tyrosinemia Type II

• Defect: Tyrosine aminotransferase

• Features:

  • Eye problems

  • Skin lesions

  • Intellectual disability may occur

Exam point

Type I → Fumarylacetoacetate hydrolase deficiency → severe liver disease

Nitisinone inhibits 4-hydroxyphenylpyruvate dioxygenase.

3. Inborn Errors of Branched-Chain & Aromatic Amino Acids

A. Branched-Chain Amino Acids (BCAAs)

The three BCAAs are:

Leucine + Isoleucine + Valine

Maple Syrup Urine Disease (MSUD)

Cause

Deficiency of:

Branched-chain α-ketoacid dehydrogenase (BCKDH)

Result

Accumulation of:

• Leucine

• Isoleucine

• Valine

• Their corresponding α-ketoacids

Features

• Sweet, maple syrup-like odor of urine

• Neurological abnormalities

• Vomiting

• Developmental delay

• Severe cases can cause encephalopathy

Treatment

• Restrict BCAAs

• Early diagnosis is important

Easy memory

MSUD = Maple Syrup + BCAAs + BCKDH deficiency

B. Aromatic Amino Acids

Important aromatic amino acids:

• Phenylalanine

• Tyrosine

• Tryptophan

Important disorders

• Amino acid           Disorder                                           Main defect

• Phenylalanine         PKU                                          Phenylalanine hydroxylase

• Tyrosine               Alkaptonuria                           Homogentisate 1,2-dioxygenase

• Tyrosine               Albinism                                                      Tyrosinase

• Tyrosine             TyrosinemiaI                                    Fumarylacetoacetate hydrolase

• Tryptophan  Disorders of serotonin metabolism          Various defects

Memory

Phe → PKU
Tyr → Albinism, Alkaptonuria, Tyrosinemia
BCAA → MSUD

4. Amino Acid-Derived Neurotransmitters and Hormones

A. Serotonin (5-HT)

Precursor

Tryptophan
↓ Tryptophan hydroxylase
5-Hydroxytryptophan (5-HTP)
↓ Aromatic L-amino acid decarboxylase
Serotonin (5-HT)

Major functions

• Mood regulation

• Sleep

• Appetite

• Pain perception

• Gastrointestinal motility

Important drug connection

• SSRIs increase serotonin activity by inhibiting its reuptake.

• Excessive serotonergic activity may cause serotonin syndrome.

Exam point

Tryptophan → Serotonin

B. Melatonin

Source

• Synthesized mainly in the pineal gland

• Derived from serotonin

Major function

• Regulates circadian rhythm

• Helps control the sleep-wake cycle

• Secretion increases in darkness

Memory

Tryptophan → Serotonin → Melatonin

C. Dopamine

Precursor

Tyrosine
↓ Tyrosine hydroxylase
L-DOPA
↓ DOPA decarboxylase
Dopamine

Major functions

• Movement control

• Reward and motivation

• Attention

• Endocrine regulation

Clinical significance

• Reduced dopamine in the substantia nigra is associated with Parkinson disease.

• Levodopa (L-DOPA) is used to increase dopamine synthesis in the brain.

Memory

Tyrosine → DOPA → Dopamine

D. Noradrenaline (Norepinephrine)

Synthesis

Dopamine
↓ Dopamine β-hydroxylase
Noradrenaline

Major functions

• Sympathetic nervous system neurotransmitter

• Increases alertness and attention

• Causes vasoconstriction

• Helps maintain blood pressure

Exam point

Dopamine → Noradrenaline

E. Adrenaline (Epinephrine)

Synthesis

Noradrenaline
↓ PNMT
Adrenaline

• PNMT = Phenylethanolamine N-methyltransferase

• Uses SAM as the methyl donor

• Stimulated by cortisol in the adrenal medulla

Major functions

• "Fight-or-flight" hormone

• Increases heart rate

• Increases cardiac contractility

• Promotes bronchodilation

• Increases blood glucose availability

Complete pathway

Tyrosine → L-DOPA → Dopamine → Noradrenaline → Adrenaline

Super-fast memory

T → D → D → N → A

Tyrosine → DOPA → Dopamine → Noradrenaline → Adrenaline

5. Catabolism of Heme

Heme is mainly derived from the breakdown of hemoglobin in old RBCs.

Main pathway

Hemoglobin
↓
Heme
↓ Heme oxygenase
Biliverdin + Fe + CO
↓ Biliverdin reductase
Unconjugated bilirubin
↓
Transported to liver bound to albumin
↓ UDP-glucuronosyltransferase (UGT1A1)
Conjugated bilirubin
↓
Bile → Intestine
↓
Urobilinogen
↓

• Stercobilin → brown color of feces

• Urobilin → yellow color of urine

Easy pathway

Heme → Biliverdin → Bilirubin → Urobilinogen → Stercobilin/Urobilin

6. Jaundice

Jaundice = Yellow discoloration of skin, sclera, and mucous membranes due to increased bilirubin.

A. Prehepatic (Hemolytic) Jaundice

Cause

• Excessive RBC destruction

• Increased heme breakdown

Main bilirubin

Unconjugated bilirubin ↑

Typical findings

• Increased bilirubin production

• Increased intestinal urobilinogen

• Increased urinary urobilinogen

• No significant bilirubinuria because unconjugated bilirubin is albumin-bound and water-insoluble

Memory

Prehepatic = Production problem

B. Hepatic Jaundice

Cause

• Liver cell damage

• Viral hepatitis

• Cirrhosis

• Other hepatic disorders

Bilirubin

• Unconjugated and/or conjugated bilirubin may increase

Memory

Hepatic = Liver processing problem

C. Posthepatic (Obstructive) Jaundice

Cause

• Obstruction of bile flow

• Gallstones

• Bile duct obstruction

Main bilirubin

Conjugated bilirubin ↑

Important features

• Dark urine

• Pale/clay-colored stools

• Decreased stercobilin formation

• Conjugated bilirubin appears in urine because it is water-soluble

Memory

Posthepatic = Passage problem

7. Important Inherited Disorders of Bilirubin Metabolism

• Disorder                                      Main defect                                                               Bilirubin

• Gilbert syndrome              Mildly reduced UGT1A1 activity                                   Unconjugated ↑

• Crigler-Najjar type I     Severe UGT1A1 deficiency                                                 Unconjugated ↑↑

• Crigler-Najjar type II   Partial UGT1A1 deficiency                                                       Unconjugated ↑

• Dubin-Johnson syndrome   Defective hepatic excretion of conjugated bilirubin       Conjugated ↑

• Rotor syndrome          Defective hepatic storage/reuptake of conjugated bilirubin    Conjugated ↑

Super-fast memory

Gilbert = Mild conjugation defect

Crigler-Najjar = Severe conjugation defect

Dubin-Johnson = Conjugated bilirubin + Black liver

Rotor = Conjugated bilirubin, no black liver

One-Minute Final Revision

Phenylalanine & Tyrosine

Phenylalanine
↓ Phenylalanine hydroxylase + BH₄
Tyrosine
↓
Fumarate + Acetoacetate

Disorders

• PKU → Phenylalanine hydroxylase ↓ → Phenylalanine ↑

• Albinism → Tyrosinase ↓ → Melanin ↓

• Alkaptonuria → Homogentisate 1,2-dioxygenase ↓ → Homogentisic acid ↑

• Tyrosinemia I → Fumarylacetoacetate hydrolase ↓ → Liver/kidney damage

• MSUD → BCKDH ↓ → Leucine + Isoleucine + Valine ↑

Amino Acid-Derived Neurotransmitters

• Tryptophan → 5-HT (Serotonin) → Melatonin

• Tyrosine → L-DOPA → Dopamine → Noradrenaline → Adrenaline

Heme Catabolism

Heme → Biliverdin → Unconjugated bilirubin → Conjugated bilirubin → Urobilinogen → Stercobilin/Urobilin

Jaundice

• Prehepatic → Unconjugated bilirubin ↑

• Hepatic → Mixed bilirubin elevation

• Posthepatic → Conjugated bilirubin ↑ + Dark urine + Pale stools

Exam Golden Rule:
"PKU = Phenylalanine, Albinism = Tyrosinase, Alkaptonuria = Homogentisate, Tyrosinemia = Tyrosine breakdown, MSUD = BCAA breakdown, Jaundice = Bilirubin metabolism."

Below is an MCQ set designed in the style of questions commonly encountered in GPAT, NIPER, AIIMS Pharmacist, Railway Pharmacist, SSC, ESIC, and State Pharmacist examinations. The questions emphasize pathway integration, enzyme defects, clinical interpretation, biochemical reasoning, and pharmacological significance rather than simple memorization.

1. A newborn develops intellectual disability, seizures, and a characteristic musty or mousy odor if untreated. The biochemical defect most directly responsible is

A. Deficiency of homogentisate 1,2-dioxygenase
B. Deficiency of phenylalanine hydroxylase
C. Deficiency of tyrosinase
D. Deficiency of fumarylacetoacetate hydrolase
E. Deficiency of branched-chain α-ketoacid dehydrogenase

Correct answer: B. Deficiency of phenylalanine hydroxylase

Brief explanation:
Phenylalanine hydroxylase converts phenylalanine → tyrosine, requiring tetrahydrobiopterin (BH₄). Its deficiency causes phenylketonuria (PKU), with accumulation of phenylalanine and formation of phenylketones.

• A → Alkaptonuria

• C → Albinism due to impaired melanin synthesis

• D → Tyrosinemia type I

• E → Maple syrup urine disease (MSUD)

2. A patient with phenylketonuria is treated with a diet restricted in phenylalanine. Which amino acid becomes conditionally essential in this patient?

A. Tryptophan
B. Tyrosine
C. Methionine
D. Histidine
E. Valine

Correct answer: B. Tyrosine

Brief explanation:
Normally, tyrosine is synthesized from phenylalanine by phenylalanine hydroxylase. In PKU, this conversion is impaired; therefore, tyrosine must be supplied through the diet.
The other amino acids do not become conditionally essential specifically because of phenylalanine hydroxylase deficiency.

3. A child with untreated phenylketonuria develops neurological impairment despite adequate caloric intake. The neurological damage is most closely associated with:

A. Excessive conversion of tyrosine to melanin
B. Inhibition of amino acid transport across the blood-brain barrier by elevated phenylalanine
C. Excessive synthesis of catecholamines
D. Accumulation of homogentisic acid in neurons
E. Increased conversion of tryptophan to serotonin

Correct answer: B. Inhibition of amino acid transport across the blood-brain barrier by elevated phenylalanine

Brief explanation:
High phenylalanine concentrations interfere with the transport of other large neutral amino acids into the brain, affecting neurotransmitter synthesis and brain development.
The other options do not explain the characteristic neurological consequences of untreated PKU.

4. A patient has normal phenylalanine hydroxylase activity but develops symptoms resembling classical phenylketonuria. The underlying defect may involve impaired regeneration of:

A. NADPH
B. Tetrahydrobiopterin (BH₄)
C. FAD
D. Pyridoxal phosphate
E. S-adenosylmethionine

Correct answer: B. Tetrahydrobiopterin (BH₄)

Brief explanation:
Phenylalanine hydroxylase requires BH₄ as a cofactor. Defects in BH₄ synthesis or regeneration can produce malignant or atypical PKU, often accompanied by impaired catecholamine and serotonin synthesis because BH₄ is also required by tyrosine hydroxylase and tryptophan hydroxylase.

5. A patient with a metabolic disorder excretes urine that becomes progressively dark on standing due to oxidation of an accumulated metabolite. The defective enzyme is most likely:

A. Phenylalanine hydroxylase
B. Tyrosinase
C. Homogentisate 1,2-dioxygenase
D. Fumarylacetoacetate hydrolase
E. Tyrosine hydroxylase

Correct answer: C. Homogentisate 1,2-dioxygenase

Brief explanation:
Alkaptonuria results from deficiency of homogentisate 1,2-dioxygenase, causing accumulation of homogentisic acid. The urine darkens upon exposure to air due to oxidation and polymerization of homogentisic acid.
The other enzymes are associated with PKU, albinism, tyrosinemia, and catecholamine synthesis, respectively.

6 An individual has reduced pigmentation of the skin, hair, and eyes but normal phenylalanine metabolism. Which enzyme defect best explains this condition?

A. Phenylalanine hydroxylase
B. Homogentisate 1,2-dioxygenase
C. Tyrosinase
D. Fumarylacetoacetate hydrolase
E. Branched-chain α-ketoacid dehydrogenase

Correct answer: C. Tyrosinase

Brief explanation:
Tyrosinase catalyzes key steps in melanin synthesis from tyrosine. Its deficiency causes oculocutaneous albinism.
PKU involves phenylalanine metabolism, while alkaptonuria and tyrosinemia involve downstream tyrosine catabolism.

7. Which sequence correctly represents the major catabolic pathway of phenylalanine and tyrosine?

A. Phenylalanine → homogentisate → tyrosine → fumarate + acetoacetate
B. Phenylalanine → tyrosine → homogentisate → fumarate + acetoacetate
C. Tyrosine → phenylalanine → homogentisate → fumarate
D. Phenylalanine → dopamine → homogentisate → fumarate
E. Phenylalanine → tryptophan → acetoacetate

Correct answer: B. Phenylalanine → tyrosine → homogentisate → fumarate + acetoacetate

Brief explanation:
Phenylalanine is first hydroxylated to tyrosine. Tyrosine is then degraded via homogentisate, ultimately yielding fumarate and acetoacetate. Therefore, tyrosine is both glucogenic and ketogenic.

8. The classification of tyrosine as both glucogenic and ketogenic is best explained by its degradation products:

A. Pyruvate and acetyl-CoA
B. Oxaloacetate and acetyl-CoA
C. Fumarate and acetoacetate
D. Succinate and propionyl-CoA
E. α-Ketoglutarate and acetoacetate

Correct answer: C. Fumarate and acetoacetate

Brief explanation:
Fumarate enters pathways that support glucose production, making tyrosine glucogenic, whereas acetoacetate is ketogenic. Thus, tyrosine is both glucogenic and ketogenic.

9. A patient with hereditary tyrosinemia type I is at increased risk of severe liver and kidney damage. Which enzyme is deficient?

A. Tyrosinase
B. Homogentisate 1,2-dioxygenase
C. Fumarylacetoacetate hydrolase
D. Phenylalanine hydroxylase
E. Tyrosine aminotransferase

Correct answer: C. Fumarylacetoacetate hydrolase

Brief explanation:
Tyrosinemia type I results from deficiency of fumarylacetoacetate hydrolase, leading to accumulation of toxic metabolites such as fumarylacetoacetate and succinylacetone. It can cause severe hepatic and renal dysfunction.

10. A patient with tyrosinemia type I is treated with nitisinone. The therapeutic rationale is to inhibit

A. Phenylalanine hydroxylase
B. Tyrosine hydroxylase
C. 4-hydroxyphenylpyruvate dioxygenase
D. Homogentisate 1,2-dioxygenase
E. Fumarylacetoacetate hydrolase

Correct answer: C. 4-Hydroxyphenylpyruvate dioxygenase

Brief explanation:
Nitisinone inhibits 4-hydroxyphenylpyruvate dioxygenase, an enzyme upstream of fumarylacetoacetate formation. This reduces the production of toxic metabolites in tyrosinemia type I. Dietary restriction of tyrosine and phenylalanine is also used.

11. A child presents with sweet-smelling urine, neurological deterioration, and elevated branched-chain amino acids. Which enzyme complex is most likely defective?

A. Pyruvate dehydrogenase complex
B. Branched-chain α-ketoacid dehydrogenase complex
C. Phenylalanine hydroxylase
D. α-Ketoglutarate dehydrogenase complex
E. Glucose-6-phosphatase

Correct answer: B. Branched-chain α-ketoacid dehydrogenase complex

Brief explanation:
Maple syrup urine disease (MSUD) results from defective degradation of the branched-chain amino acids leucine, isoleucine, and valine. The defect occurs in the branched-chain α-ketoacid dehydrogenase complex.

12. Which amino acid is exclusively ketogenic among the following?

A. Valine
B. Leucine
C. Phenylalanine
D. Tyrosine
E. Isoleucine

Correct answer: B. Leucine

Brief explanation:
Leucine and lysine are the two exclusively ketogenic amino acids.

• Valine → glucogenic

• Isoleucine → both glucogenic and ketogenic

• Phenylalanine and tyrosine → both glucogenic and ketogenic

13. A patient with maple syrup urine disease is expected to have elevated levels of which combination?

A. Phenylalanine, tyrosine, and tryptophan
B. Leucine, isoleucine, and valine
C. Lysine, arginine, and histidine
D. Methionine, cysteine, and serine
E. Alanine, glycine, and glutamate

Correct answer: B. Leucine, isoleucine, and valine

Brief explanation:
MSUD affects the metabolism of the three branched-chain amino acids (BCAAs): leucine, isoleucine, and valine. Their corresponding α-ketoacids also accumulate.

14. Which amino acid is the immediate precursor of serotonin?

A. Tyrosine
B. Phenylalanine
C. Tryptophan
D. Histidine
E. Methionine

Correct answer: C. Tryptophan

Brief explanation:
Serotonin (5-hydroxytryptamine, 5-HT) is synthesized from tryptophan through hydroxylation followed by decarboxylation. Tyrosine is the precursor of catecholamines, whereas histidine gives rise to histamine.

15. A patient taking a selective serotonin reuptake inhibitor (SSRI) develops agitation, hyperreflexia, autonomic instability, and clonus after adding another serotonergic drug. The biochemical basis is

A. Excessive dopamine degradation
B. Excessive serotonin activity in the CNS
C. Decreased melatonin synthesis
D. Increased acetylcholine breakdown
E. Inhibition of norepinephrine synthesis

Correct answer: B. Excessive serotonin activity in the CNS

Brief explanation:
The clinical picture is characteristic of serotonin syndrome, caused by excessive serotonergic activity. SSRIs increase serotonin signaling primarily by inhibiting serotonin reuptake. The other options do not account for the characteristic neuromuscular and autonomic findings.

16. The rate-limiting enzyme in serotonin synthesis is:

A. Aromatic L-amino acid decarboxylase
B. Tryptophan hydroxylase
C. Tyrosine hydroxylase
D. Monoamine oxidase
E. Catechol-O-methyltransferase

Correct answer: B. Tryptophan hydroxylase

Brief explanation:
Serotonin synthesis proceeds as

Tryptophan → 5-hydroxytryptophan → serotonin

Tryptophan hydroxylase catalyzes the rate-limiting hydroxylation step. Aromatic L-amino acid decarboxylase catalyzes the subsequent decarboxylation.

17. Melatonin is synthesized primarily in the pineal gland from serotonin. Its major physiological role is regulation of:

A. Blood glucose concentration
B. Circadian rhythm and sleep-wake cycle
C. Calcium homeostasis
D. Thyroid hormone synthesis
E. Renal sodium excretion

Correct answer: B. Circadian rhythm and sleep-wake cycle

Brief explanation:
Melatonin is synthesized from serotonin in the pineal gland and plays an important role in regulating circadian rhythms and sleep timing. Its secretion generally increases during darkness.

18. A patient with Parkinson's disease has reduced dopamine production in the substantia nigra. Dopamine is synthesized from tyrosine through which correct sequence?

A. Tyrosine → DOPA → dopamine → norepinephrine → epinephrine
B. Tyrosine → dopamine → DOPA → norepinephrine → epinephrine
C. Tyrosine → serotonin → dopamine → norepinephrine
D. Phenylalanine → DOPA → dopamine → serotonin
E. Tyrosine → DOPA → serotonin → epinephrine

Correct answer: A. Tyrosine → DOPA → dopamine → norepinephrine → epinephrine

Brief explanation:
Catecholamine synthesis follows:

Tyrosine → L-DOPA → Dopamine → Norepinephrine → Epinephrine

Tyrosine hydroxylase is the rate-limiting enzyme. Levodopa (L-DOPA) is used therapeutically in Parkinson's disease because it crosses the blood-brain barrier more effectively than dopamine.

19. Which enzyme converts dopamine to norepinephrine?

A. Tyrosine hydroxylase
B. DOPA decarboxylase
C. Dopamine β-hydroxylase
D. Phenylethanolamine N-methyltransferase
E. Monoamine oxidase

Correct answer: C. Dopamine β-hydroxylase

Brief explanation:
Dopamine is converted to norepinephrine by dopamine β-hydroxylase.

• Tyrosine hydroxylase → tyrosine to L-DOPA

• DOPA decarboxylase → L-DOPA to dopamine

• PNMT → norepinephrine to epinephrine

• MAO → degradation of catecholamines

20. A patient experiences increased heart rate, cardiac contractility, and bronchodilation during an acute stress response. The principal catecholamine responsible for these effects is

A. Dopamine
B. Serotonin
C. Epinephrine
D. Melatonin
E. GABA

Correct answer: C. Epinephrine

Brief explanation:
Epinephrine (adrenaline) is a major hormone of the adrenal medulla and mediates the "fight-or-flight" response. It increases cardiac activity and promotes bronchodilation. Norepinephrine is more strongly associated with vasoconstriction and maintenance of vascular tone.

21. The conversion of norepinephrine to epinephrine in the adrenal medulla requires:

A. Tyrosine hydroxylase
B. DOPA decarboxylase
C. Dopamine β-hydroxylase
D. Phenylethanolamine N-methyltransferase
E. Monoamine oxidase

Correct answer: D. Phenylethanolamine N-methyltransferase (PNMT)

Brief explanation:
PNMT converts norepinephrine to epinephrine using S-adenosylmethionine (SAM) as the methyl donor. Cortisol from the adrenal cortex induces PNMT expression in the adrenal medulla.

22. Which amino acid-derived neurotransmitter is synthesized from tyrosine and is also a precursor of norepinephrine and epinephrine?

A. Serotonin
B. Dopamine
C. Melatonin
D. Histamine
E. GABA

Correct answer: B. Dopamine

Brief explanation:
Dopamine is a catecholamine synthesized from tyrosine. It is further converted to norepinephrine and then epinephrine. Serotonin and melatonin are derived from tryptophan.

23. A patient with severe liver disease develops jaundice. Laboratory findings show increased unconjugated bilirubin. Which process is most likely impaired?

A. Conversion of heme to biliverdin
B. Conversion of biliverdin to bilirubin
C. Hepatic conjugation of bilirubin with glucuronic acid
D. Conversion of bilirubin to urobilinogen by intestinal bacteria
E. Renal excretion of urobilinogen

Correct answer: C. Hepatic conjugation of bilirubin with glucuronic acid

Brief explanation:
Unconjugated bilirubin is transported to the liver bound to albumin and is conjugated with glucuronic acid by UDP-glucuronosyltransferase (UGT1A1). Impaired hepatic uptake or conjugation increases unconjugated bilirubin.

24. A patient has severe hemolysis and develops jaundice. Which biochemical pattern is most likely?

A. Increased conjugated bilirubin with decreased bilirubin production
B. Increased unconjugated bilirubin due to excessive heme breakdown
C. Decreased urobilinogen formation
D. Increased direct bilirubin due to bile duct obstruction
E. Complete absence of bilirubin in plasma

Correct answer: B. Increased unconjugated bilirubin due to excessive heme breakdown

Brief explanation:
In hemolytic (prehepatic) jaundice, excessive destruction of erythrocytes increases heme breakdown and bilirubin production. The liver may be unable to conjugate the increased bilirubin load efficiently, resulting in increased unconjugated bilirubin.

25. A patient with obstructive jaundice has pale stools and dark urine. Which biochemical finding is most consistent with this condition?

A. Increased urinary unconjugated bilirubin
B. Increased urinary conjugated bilirubin
C. Complete absence of bilirubin in plasma
D. Decreased plasma conjugated bilirubin
E. Increased fecal stercobilin

Correct answer: B. Increased urinary conjugated bilirubin

Brief explanation:
In posthepatic obstructive jaundice, conjugated bilirubin cannot efficiently enter the intestine and instead re-enters the bloodstream. Because conjugated bilirubin is water-soluble, it appears in urine, causing dark urine. Reduced intestinal bilirubin delivery decreases stercobilin formation, resulting in pale stools.

26. Which sequence correctly represents the major pathway of heme degradation?

A. Heme → bilirubin → biliverdin → urobilinogen
B. Heme → biliverdin → bilirubin → bilirubin diglucuronide
C. Heme → urobilinogen → biliverdin → bilirubin
D. Heme → bilirubin → urobilin → biliverdin
E. Heme → biliverdin → urobilin → bilirubin

Correct answer: B. Heme → biliverdin → bilirubin → bilirubin diglucuronide

Brief explanation:
Heme is converted to biliverdin by heme oxygenase, releasing iron and carbon monoxide. Biliverdin is then reduced to bilirubin by biliverdin reductase. In the liver, bilirubin is conjugated with glucuronic acid.

27. The conversion of heme to biliverdin is unusual because it produces which additional molecule?

A. Nitric oxide
B. Carbon monoxide
C. Hydrogen sulfide
D. Ammonia
E. Methane

Correct answer: B. Carbon monoxide

Brief explanation:
Heme oxygenase catalyzes heme degradation to biliverdin and releases iron and carbon monoxide (CO). This is a distinctive biochemical feature of heme catabolism.

28. A newborn develops severe unconjugated hyperbilirubinemia due to a nearly complete deficiency of bilirubin UDP-glucuronosyltransferase. The condition is most consistent with:

A. Gilbert syndrome
B. Crigler-Najjar syndrome type I
C. Dubin-Johnson syndrome
D. Rotor syndrome
E. Hemolytic anemia

Correct answer: B. Crigler-Najjar syndrome type I

Brief explanation:
Crigler-Najjar syndrome type I involves severe deficiency of UGT1A1, producing marked unconjugated hyperbilirubinemia and a high risk of kernicterus.
Gilbert syndrome is much milder. Dubin-Johnson and Rotor syndromes primarily cause conjugated hyperbilirubinemia.

29. A patient has mild intermittent unconjugated hyperbilirubinemia, particularly during fasting or stress, with otherwise normal liver function. The most likely diagnosis is:

A. Gilbert syndrome
B. Crigler-Najjar syndrome type I
C. Dubin-Johnson syndrome
D. Obstructive jaundice
E. Acute viral hepatitis

Correct answer: A. Gilbert syndrome

Brief explanation:
Gilbert syndrome is a benign condition caused by reduced bilirubin conjugation capacity due to decreased UGT1A1 activity. Episodes may become more apparent during fasting, illness, or stress.

30. A patient has conjugated hyperbilirubinemia and a characteristically darkly pigmented liver. Which disorder is most likely?

A. Gilbert syndrome
B. Crigler-Najjar syndrome
C. Dubin-Johnson syndrome
D. Hemolytic jaundice
E. Phenylketonuria

Correct answer: C. Dubin-Johnson syndrome

Brief explanation:
Dubin-Johnson syndrome is an inherited disorder of hepatic excretion of conjugated bilirubin, associated with deposition of dark pigment in hepatocytes and a black liver.
Gilbert and Crigler-Najjar primarily cause unconjugated hyperbilirubinemia.

31. A patient with hemolytic anemia has increased bilirubin production. Which compound is expected to increase in the intestine as a consequence of increased bilirubin delivery?

A. Biliverdin
B. Bilirubin diglucuronide
C. Urobilinogen
D. Hemoglobin
E. Heme

Correct answer: C. Urobilinogen

Brief explanation:
Increased heme breakdown increases bilirubin production. More bilirubin reaches the intestine, where bacterial metabolism produces urobilinogen. Increased intestinal urobilinogen formation contributes to increased urinary urobilinogen in hemolytic jaundice.

32. A patient with severe obstructive jaundice has almost no bilirubin reaching the intestine. Which finding is expected?

A. Increased fecal stercobilin
B. Increased urinary unconjugated bilirubin
C. Decreased or absent fecal stercobilin
D. Increased intestinal urobilinogen production
E. Increased hemoglobin degradation in the intestine

Correct answer: C. Decreased or absent fecal stercobilin

Brief explanation:
Bile obstruction prevents conjugated bilirubin from reaching the intestine. Consequently, less bilirubin is converted to urobilinogen and stercobilin, producing pale or clay-colored stools.

33. A researcher inhibits heme oxygenase in a patient. Which immediate biochemical conversion would be directly reduced?

A. Biliverdin formation from heme
B. Bilirubin conjugation
C. Urobilinogen formation
D. Stercobilin formation
E. Bilirubin oxidation to biliverdin

Correct answer: A. Biliverdin formation from heme

Brief explanation:
Heme oxygenase catalyzes the first major step in heme degradation, converting heme to biliverdin while releasing iron and carbon monoxide. Inhibition therefore reduces biliverdin formation directly.

34. Which pair correctly matches the metabolic disorder with its defective enzyme?

A. Phenylketonuria — Tyrosinase
B. Alkaptonuria — Homogentisate 1,2-dioxygenase
C. Albinism — Phenylalanine hydroxylase
D. MSUD — Phenylalanine hydroxylase
E. Tyrosinemia type I — Tyrosinase

Correct answer: B. Alkaptonuria — Homogentisate 1,2-dioxygenase

Brief explanation:
This is the correct enzyme-disorder association.

• PKU → phenylalanine hydroxylase deficiency

• Albinism → commonly tyrosinase deficiency in classic biochemical teaching

• MSUD → branched-chain α-ketoacid dehydrogenase deficiency

• Tyrosinemia type I → fumarylacetoacetate hydrolase deficiency

35. A patient has a defect affecting the synthesis of tetrahydrobiopterin (BH₄). Which combination of biochemical pathways could be simultaneously impaired?

A. Urea cycle and heme synthesis
B. Serotonin and catecholamine synthesis
C. Glycolysis and β-oxidation
D. Heme degradation and bilirubin conjugation
E. Purine degradation and pyrimidine synthesis

Correct answer: B. Serotonin and catecholamine synthesis

Brief explanation:
BH₄ is required by hydroxylation reactions catalyzed by phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase. Therefore, BH₄ deficiency can impair phenylalanine metabolism as well as synthesis of catecholamines and serotonin.

Conceptual Takeaways

• Phenylalanine → Tyrosine: Phenylalanine hydroxylase + BH₄

• PKU: Phenylalanine hydroxylase or BH₄-related defects

• Tyrosine → Fumarate + Acetoacetate: Both glucogenic and ketogenic

• Alkaptonuria: Homogentisate 1,2-dioxygenase deficiency

• Albinism: Defective melanin synthesis, classically associated with tyrosinase deficiency

• Tyrosinemia type I: Fumarylacetoacetate hydrolase deficiency; treated with nitisinone

• MSUD: Branched-chain α-ketoacid dehydrogenase deficiency

• Leucine and lysine: Exclusively ketogenic

• Tryptophan → Serotonin → Melatonin

• Tyrosine → L-DOPA → Dopamine → Norepinephrine → Epinephrine

• Heme → Biliverdin → Bilirubin → Conjugated bilirubin → Urobilinogen → Stercobilin/Urobilin

• Prehepatic jaundice: Predominantly unconjugated bilirubin

• Hepatic jaundice: Mixed pattern may occur

• Posthepatic/obstructive jaundice: Predominantly conjugated bilirubin, dark urine, pale stools

• Gilbert syndrome: Mild unconjugated hyperbilirubinemia

• Crigler-Najjar syndrome: Severe unconjugated hyperbilirubinemia

• Dubin-Johnson syndrome: Conjugated hyperbilirubinemia + black liver

Dr. Alok Singh