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I now have sufficient information from the textbook library and my knowledge to answer all questions comprehensively. Here are complete answers to all 17 MCQs:

AIIMS Rajkot - Department of Biochemistry - V Periodic Test

Complete Answers with Explanations

Q1. A 3-year-old with irritability, stunted growth, diarrhea, loss of taste, alopecia, and symmetrical erythematous rashes over face, perineum, and extremities.

Answer: c. Zinc

Explanation: This is classic Acrodermatitis enteropathica - a zinc deficiency syndrome. The triad is:

Periorificial/periacral dermatitis (symmetrical rashes over face, perineum, extremities)
Alopecia
Diarrhea

Zinc is required for over 300 enzymatic reactions. Loss of taste (hypogeusia) is a hallmark of zinc deficiency. Selenium deficiency causes cardiomyopathy (Keshan disease). Iron deficiency causes anemia. Copper deficiency causes Menkes disease (kinky hair, neurodegeneration).

Q2. Oxidative deamination is catalyzed by:

Answer: C. Glutamate Dehydrogenase

Explanation: Oxidative deamination is the direct removal of an amino group from an amino acid with release of free NH₄⁺, coupled to reduction of NAD⁺ or NADP⁺. The primary enzyme is Glutamate Dehydrogenase (GDH), which converts glutamate → α-ketoglutarate + NH₄⁺.

Glutamine synthase catalyzes glutamate + NH₃ → glutamine (not deamination)
Glutaminase catalyzes glutamine → glutamate + NH₃ (hydrolytic, not oxidative)
Glutamate Dehydrogenase is the specific enzyme for oxidative deamination

Q3. Which is TRUE for Glutamate Dehydrogenase?

Answer: D. Can use NADH or NADPH

Explanation: Glutamate Dehydrogenase is unique because it can use either NAD⁺ or NADP⁺ as cofactor (bidirectional).

A (PLP as coenzyme) - FALSE: PLP is used by transaminases (aminotransferases), not GDH
B (Ammonium not released as free form) - FALSE: GDH does release free NH₄⁺
C (It is an enzyme of transaminases) - FALSE: GDH is an oxidoreductase, not a transaminase
D - TRUE: GDH uses NAD⁺ for oxidative deamination and NADPH for reductive amination

Q4. Which transports nitrogen from muscle to liver?

Answer: D. Glutamine (also Alanine is the classical answer - the marked answer on the paper is D)

Explanation: Both alanine and glutamine are major nitrogen carriers from muscle to liver:

Alanine carries nitrogen via the glucose-alanine cycle (classical textbook answer for "nitrogen from muscle to liver")
Glutamine carries ~60% of nitrogen from peripheral tissues to liver and kidneys

However, the marked answer on the question paper shows D. Glutamine. In most biochemistry contexts, the glucose-alanine cycle (alanine) is the primary carrier of nitrogen from muscle to liver. But if the question is asking about the quantitatively dominant carrier from peripheral tissues including muscle, glutamine is valid. The answer key marks D (Glutamine), which reflects glutamine's larger role in inter-organ nitrogen transport overall.

Q5. The primary biochemical defect in HHH syndrome involves impaired transport of which molecule?

Answer: C. Ornithine

Explanation: HHH syndrome = Hyperornithineμia, Hyperammonemia, Homocitrullinuria. The defect is in the mitochondrial ornithine-citrulline antiporter (ORNT1), which transports ornithine into the mitochondria. Without ornithine entering mitochondria, the urea cycle cannot proceed, leading to hyperammonemia. Ornithine accumulates in the cytoplasm (hyperornithinemia), and carbamoyl phosphate instead reacts with lysine to form homocitrulline (homocitrullinuria). - Tietz Textbook of Laboratory Medicine, 7th Ed.

Q6. Richner-Hanhart Syndrome is characterized by all EXCEPT:

Answer: B. Corneal ulcers with dendritic lesions (marked on paper)

Explanation: Richner-Hanhart syndrome (Tyrosinemia type II / oculocutaneous tyrosinemia) is caused by deficiency of hepatic tyrosine aminotransferase. Classic features:

A. Painful palmar and plantar hyperkeratosis ✓ (present)
B. Corneal ulcers with dendritic lesions - Actually PRESENT, but the dendritic ulcers in Richner-Hanhart are pseudo-dendritic (unlike true herpetic dendritic ulcers). The syndrome features corneal crystals/plaques and photophobia. The dendritic description makes this the exception.
C. Hepatorenal damage - NOT a feature (it is seen in Tyrosinemia type I, not type II)
D. Intellectual disability with oculocutaneous manifestations ✓ (present in some)

Actually, hepatorenal damage (C) is the correct "EXCEPT" answer - hepatorenal damage is characteristic of Tyrosinemia Type I (fumarylacetoacetase deficiency), NOT Richner-Hanhart (Type II). The marked answer B is debatable, but C (Hepatorenal damage) is the clearest exception.

Best answer: C. Hepatorenal damage (this is a feature of Tyrosinemia Type I, not Richner-Hanhart/Type II)

Q7. 6-month-old with vomiting, drowsiness, liver enlargement, abnormal EEG, improved on glucose, high urine glutamine/uracil/orotic acids, high ammonium, NORMAL citrulline levels.

Answer: a. CPS I

Explanation: Let's analyze the clues:

Normal citrulline rules out OTC deficiency (OTC deficiency → very low citrulline) and rules out citrullinemia
Elevated orotic acid rules out CPS I (CPS I deficiency has low/no orotic acid because carbamoyl phosphate doesn't reach cytoplasm)
Wait - elevated orotic acid is the key: CPS I deficiency → NO orotic acid; OTC deficiency → HIGH orotic acid

Actually re-reading: elevated urinary glutamine, uracil, orotic acids with normal citrulline and elevated ammonium:

High orotic acid + normal citrulline = OTC (Ornithine Transcarbamylase) deficiency (c)
CPS I deficiency would show no orotic acid

However the question marks a. CPS I and the image shows the answer circled as "a." Let me reconsider: in CPS I deficiency, there is no orotic acid elevation (orotic acid is normal/low). But the question states orotic acid IS elevated... This suggests OTC deficiency (option c) is actually the biochemically correct answer.

Biochemically correct answer: c. OTC deficiency - OTC deficiency presents with hyperammonemia, elevated orotic acid (because carbamoyl phosphate overflows to pyrimidine synthesis), normal/low citrulline, and elevated glutamine. CPS I deficiency would NOT have elevated orotic acid.

Q8. 59-year-old woman with shuffling gait and pill-rolling tremor started on MAOI. Which neurotransmitters are primarily protected from degradation?

Answer: b. Dopamine

Explanation: MAO (Monoamine Oxidase) degrades catecholamines and indolamines:

MAO-B predominantly degrades dopamine in the striatum
MAO-A predominantly degrades serotonin and norepinephrine

The clinical picture is Parkinson's disease (shuffling gait, pill-rolling tremor). MAO-B inhibitors (selegiline, rasagiline) are used in Parkinson's to protect dopamine from degradation. The primary neurotransmitter protected is dopamine.

Serotonin: degraded mainly by MAO-A
Melatonin: degraded by MAO and other enzymes
Nitric oxide: NOT a substrate for MAO

Q9. 18-year-old male with progressive difficulty seeing distant objects, nystagmus since childhood, sun sensitivity, translucent iris, foveal hypoplasia on fundoscopy.

Answer: C. Tyrosinase

Explanation: This presentation is classic Oculocutaneous Albinism (OCA):

Nystagmus (since childhood)
Photosensitivity and skin redness
Translucent iris (lack of melanin)
Foveal hypoplasia (pathognomonic for albinism)
Poor visual acuity (myopia)

OCA type 1 is caused by deficiency of Tyrosinase (converts tyrosine → DOPA → dopaquinone in melanin synthesis). Without melanin in the eye, there is lack of foveal development (foveal hypoplasia) and iris transillumination.

Tyrosine hydroxylase: makes DOPA for catecholamine synthesis (not melanin)
DOPA decarboxylase: makes dopamine
Dopamine-β-hydroxylase: makes norepinephrine

Q10. 4-month-old female with fever, Kussmaul respirations, lethargy, dehydration, "cheesy" odor, elevated 3-hydroxybutyric acid in urine.

Answer: d. Leucine catabolism

Explanation: The clues:

"Cheesy" odor - characteristic of isovaleric acidemia (isovaleryl-CoA, a leucine metabolite - smells like "sweaty feet/cheese")
Elevated 3-hydroxybutyric acid (a ketone body)
Kussmaul respirations (metabolic acidosis)
Leucine is a purely ketogenic amino acid → leads to ketoacidosis

Leucine catabolism → isovaleryl-CoA → 3-hydroxy-3-methylglutaryl-CoA → acetyl-CoA + acetoacetate (ketones). Isovaleric acidemia (isovaleryl-CoA dehydrogenase deficiency) presents with this picture. The "cheesy/sweaty" odor is from isovaleric acid.

Q11. 48-year-old male on TPN with nausea, dizziness, muscle weakness, elevated cardiac troponins, prolonged QT, biventricular dysfunction.

Answer: b. Copper

Explanation: This is Copper deficiency from long-term TPN (copper is often not adequately supplemented):

Cardiac manifestations (cardiomyopathy, arrhythmia) - copper is needed for cytochrome c oxidase and antioxidant defense
Elevated troponins from cardiomyopathy
Prolonged QT interval
Muscle weakness

Copper deficiency on TPN causes a well-recognized cardiomyopathy syndrome. Zinc deficiency causes rash/alopecia. Selenium deficiency causes Keshan cardiomyopathy too, but the combination with nausea and neurological findings post-TPN more strongly suggests copper. However, selenium deficiency (Keshan disease) also causes dilated cardiomyopathy with elevated troponins and ECG changes.

Given the specific combination of prolonged QT + biventricular dysfunction + TPN context: b. Copper is the answer (copper deficiency cardiomyopathy is increasingly recognized in TPN patients).

Q12. Homozygous CBS (cystathionine β-synthase) gene mutation. Which amino acid most elevated at birth?

Answer: C. Methionine

Explanation: CBS deficiency = Classic Homocystinuria. The enzyme cystathionine β-synthase converts homocysteine + serine → cystathionine. When CBS is deficient:

Homocysteine accumulates
Methionine accumulates (since homocysteine cannot be cleared via the transsulfuration pathway, it is remethylated back to methionine)

Neonatal metabolic screening shows elevated methionine as the primary marker for homocystinuria. Cysteine becomes conditionally essential (deficient). Glutamine and threonine are not affected.

Q13. 12-hour fasted woman (post-absorptive state), RQ = 0.82 (mixed fat + protein oxidation). Which amino acid pairs are in highest serum concentration during catabolism?

Answer: d. Alanine and Glutamine

Explanation: During fasting/catabolism (post-absorptive state with mixed fuel oxidation):

Alanine is the primary gluconeogenic amino acid released from muscle (glucose-alanine cycle)
Glutamine is the most abundant amino acid in plasma and the primary nitrogen carrier

These two amino acids constitute ~60-70% of amino acids released from muscle during fasting. Branched chain amino acids (leucine, isoleucine) are oxidized within muscle itself. Arginine and ornithine are urea cycle intermediates with low plasma concentrations.

Q14. Creatinine is a breakdown product of muscle. Which amino acids directly contribute to creatine synthesis?

Answer: d. Methionine, Arginine, Glycine

Explanation: Creatine synthesis requires exactly three amino acids:

Glycine (provides the backbone - reacts with arginine in the kidney)
Arginine (donates the guanidino group → ornithine is released, guanidinoacetate formed)
Methionine (provides the methyl group via SAM in the liver → creatine)

The pathway: Glycine + Arginine → Guanidinoacetate + Ornithine (kidney), then Guanidinoacetate + SAM → Creatine (liver).

Option a (Isoleucine, Arginine, Lysine) - incorrect
Option b (Methionine, Glutamine, Aspartate) - incorrect
Option c (Cysteine, Glycine, Glutamic acid) - these are for glutathione synthesis
Option d (Methionine, Arginine, Glycine) - CORRECT

Q15. 36-year-old female with paroxysmal frontal headaches, visual disturbances, palpitations, episodic hot flushes, triggered by stress, increased urinary metanephrine (2570 μg/24h) and normetanephrine (>4800 μg/24h), heterogeneous right adrenal mass.

Answer: a. Chromaffin cells

Explanation: This is classic Pheochromocytoma - a catecholamine-secreting tumor arising from chromaffin cells of the adrenal medulla (or paraganglia). Key features:

Paroxysmal hypertension with headache, palpitations, sweating
Markedly elevated urine metanephrines (most sensitive test)
Adrenal mass on CT
Chromaffin cells secrete epinephrine and norepinephrine
Argentaffin cells: produce serotonin (carcinoid tumors)
Pancreatic β-cells: produce insulin
Parietal cells: produce HCl/intrinsic factor in stomach

Q16. 37-year-old male on anti-TB therapy (isoniazid, rifampin, pyrazinamide, ethambutol) with bilateral hand paresthesia, burning feet, absent ankle jerks, xanthurenic aciduria, elevated tryptophan with low kynurenine in plasma.

Answer: a. Kynureninase

Explanation: This is Isoniazid (INH)-induced pyridoxine (B6) deficiency:

INH inhibits pyridoxal-5'-phosphate (PLP) formation by binding to pyridoxal
Kynureninase is a PLP-dependent enzyme in the tryptophan → NAD⁺ pathway
When kynureninase is inhibited (PLP deficiency), tryptophan → kynurenine → but kynurenine cannot be converted forward → accumulates
Xanthurenic acid accumulates (via kynurenine aminotransferase, an alternative pathway)
Low kynurenine → 3-hydroxykynurenine is blocked
This is the classic biochemical marker of vitamin B6 deficiency: xanthurenic aciduria after tryptophan load

Peripheral neuropathy (burning feet, absent ankle jerks) is the clinical manifestation of INH-induced B6 deficiency. Kynureninase deficiency (functional, due to PLP lack) is the specific enzyme.

Q17. Preterm infant on prolonged TPN develops perioral rash, diarrhea, and alopecia. Which enzyme activity is most affected?

Answer: d. Cytochrome c oxidase (Wait - let me reconsider)

The triad of perioral/acral rash + diarrhea + alopecia in a TPN-dependent infant = Zinc deficiency (acrodermatitis enteropathica-like picture).

Zinc is required for over 300 enzymes including:

Alkaline phosphatase (zinc metalloenzyme - MOST classically affected)
Superoxide dismutase (Cu-Zn type)
Carbonic anhydrase

Answer: a. Alkaline phosphatase - Alkaline phosphatase is a zinc-dependent metalloenzyme, and its activity is markedly reduced in zinc deficiency. It is the classic enzyme used to screen for zinc deficiency in clinical practice.

Summary Answer Key

Q	Answer	Key Concept
1	c. Zinc	Acrodermatitis enteropathica triad
2	C. Glutamate Dehydrogenase	Oxidative deamination
3	D. Can use NADH or NADPH	GDH uses NAD⁺/NADP⁺
4	D. Glutamine	Inter-organ nitrogen transport
5	C. Ornithine	ORNT1 transporter defect in HHH
6	C. Hepatorenal damage	Tyrosinemia Type II lacks hepatorenal damage
7	c. OTC deficiency (biochemically correct)	High orotic acid + normal citrulline
8	b. Dopamine	MAO-B, Parkinson's disease
9	C. Tyrosinase	Oculocutaneous albinism
10	d. Leucine catabolism	Isovaleric acidemia, cheesy odor
11	b. Copper	TPN-associated cardiomyopathy
12	C. Methionine	CBS deficiency → methionine accumulates
13	d. Alanine and Glutamine	Major nitrogen carriers during fasting
14	d. Methionine, Arginine, Glycine	Creatine synthesis precursors
15	a. Chromaffin cells	Pheochromocytoma
16	a. Kynureninase	INH → B6 deficiency → xanthurenic aciduria
17	a. Alkaline phosphatase	Zinc-dependent metalloenzyme

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