Amino Acid Metabolism Practice Questions

Q: Which of the following is NOT an enzyme of the urea cycle?

Citrulline synthase. The urea cycle (Krebs-Henseleit cycle) occurs in the liver to convert toxic ammonia into water-soluble urea. The correct answer is **Citrulline synthase** because no such enzyme exists in the urea cycle; Citrulline is actually synthesized by the enzyme **Ornithine transcarbamylase (OTC)**. ### Why Citrulline synthase is the correct answer: In the second step of the urea cycle, Carbamoyl phosphate reacts with Ornithine to form Citrulline. This reaction is catalyzed by **Ornithine transcarbamylase (OTC)**, not "Citrulline synthase." This is a common distractor in biochemistry exams. ### Analysis of incorrect options: * **A. Ornithine transcarbamylase:** This is a mitochondrial enzyme that catalyzes the formation of Citrulline. It is the most common enzyme deficiency in the urea cycle (X-linked recessive). * **B. Carbamoyl-phosphate synthetase I (CPS-I):** This is the **rate-limiting enzyme** of the urea cycle, located in the mitochondria. It requires N-acetylglutamate (NAG) as an essential allosteric activator. * **C. Argininosuccinase (Argininosuccinate lyase):** This cytosolic enzyme cleaves Argininosuccinate into Arginine and Fumarate. Fumarate then provides a link to the TCA cycle (the "Krebs bicycle"). ### High-Yield Clinical Pearls for NEET-PG: * **Location:** The cycle is "split"—the first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **Mnemonic for Enzymes:** **C**an **O**rdinary **A**nyone **A**lways **A**nywhere (**C**PS-I, **O**TC, **A**rgininosuccinate synthetase, **A**rgininosuccinate lyase, **A**rginase). * **Arginase:** The final enzyme that releases urea and regenerates Ornithine. It is found almost exclusively in the liver. * **Hyperammonemia Type II:** Caused by OTC deficiency; characterized by increased orotic acid in urine.

Q: Diagnosis of carcinoid tumor is done by?

5-HIAA. **Explanation:** **1. Why 5-HIAA is the correct answer:** Carcinoid tumors are neuroendocrine tumors, most commonly found in the gastrointestinal tract (ileum) and lungs. These tumors frequently secrete excessive amounts of **Serotonin (5-hydroxytryptamine)**. Serotonin is metabolized by the enzyme monoamine oxidase (MAO) and aldehyde dehydrogenase into **5-Hydroxyindoleacetic acid (5-HIAA)**, which is then excreted in the urine. Therefore, a **24-hour urinary 5-HIAA test** is the gold standard biochemical marker for diagnosing and monitoring carcinoid syndrome. **2. Why the other options are incorrect:** * **DHEA (Dehydroepiandrosterone):** This is an androgen precursor produced by the adrenal cortex. It is used as a marker for adrenal tumors or polycystic ovary syndrome (PCOS), not carcinoid tumors. * **VMA (Vanillylmandelic Acid):** This is the end-stage metabolite of catecholamines (epinephrine and norepinephrine). It is used primarily to diagnose **Pheochromocytoma** and Neuroblastoma. * **Metanephrines:** These are intermediate metabolites of catecholamines. Urinary or plasma metanephrines are currently considered the most sensitive screening test for **Pheochromocytoma**. **3. Clinical Pearls for NEET-PG:** * **Dietary Caution:** Patients must avoid serotonin-rich foods (bananas, walnuts, pineapples, avocados) 24–48 hours before the 5-HIAA test to prevent false positives. * **Pellagra Risk:** In carcinoid syndrome, up to 60% of dietary **Tryptophan** is diverted to serotonin synthesis, leading to a deficiency in Niacin (Vitamin B3) production. This can result in **Pellagra** (Dermatitis, Diarrhea, Dementia). * **Localization:** The most common site for a carcinoid tumor is the **appendix**, but the most common site to cause "Carcinoid Syndrome" (due to metastasis to the liver) is the **ileum**.

Q: Which amino acid plays a role in the sleep-wake cycle?

Tryptophan. **Explanation:** **Tryptophan** is the correct answer because it is the primary precursor for **Serotonin** and **Melatonin**, the two key neurotransmitters regulating the sleep-wake cycle (circadian rhythm). 1. **Mechanism:** Tryptophan is converted to 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase, which is then decarboxylated to Serotonin. In the pineal gland, Serotonin undergoes N-acetylation and O-methylation to form **Melatonin**. Melatonin levels rise in darkness, signaling the body to initiate sleep. 2. **Incorrect Options:** * **Phenylalanine:** A precursor to Tyrosine, which leads to the synthesis of catecholamines (Dopamine, Epinephrine, Norepinephrine). These are generally associated with alertness and the "fight or flight" response rather than sleep induction. * **Glycine:** An inhibitory neurotransmitter in the spinal cord and brainstem. While it has some calming effects, it is not the primary regulator of the circadian rhythm. * **Glutamine:** Primarily acts as a non-toxic carrier of ammonia and a precursor to Glutamate (excitatory) and GABA (inhibitory). It does not directly synthesize melatonin. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Tryptophan hydroxylase is the rate-limiting enzyme for serotonin synthesis. * **Vitamin Cofactors:** The conversion of Tryptophan to Serotonin requires **Vitamin B6 (Pyridoxine)** and **Tetrahydrobiopterin (BH4)**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) can lead to niacin deficiency (Pellagra-like symptoms) because Tryptophan is also a precursor for **Niacin (Vitamin B3)**. * **Melatonin Synthesis:** Occurs specifically in the **Pineal Gland**; its secretion is inhibited by light hitting the retina.

Q: In phaeochromocytoma, what substance will be present in elevated amounts in the urine?

Vanillylmandelic acid (VMA). PHAEOCROMOCYTOMA EXPLANATION: **1. Why Vanillylmandelic acid (VMA) is correct:** Pheochromocytoma is a catecholamine-secreting tumor arising from the chromaffin cells of the adrenal medulla [1]. These tumors overproduce epinephrine and norepinephrine. These catecholamines are metabolized by enzymes (COMT and MAO) into intermediate metabolites called **metanephrines** and finally into the end-product, **Vanillylmandelic acid (VMA)**. Because these substances are excreted by the kidneys, elevated levels of urinary VMA (measured via a 24-hour urine collection) serve as a classic biochemical marker for diagnosing pheochromocytoma. **2. Why the other options are incorrect:** * **5-hydroxyindoleacetic acid (5-HIAA):** This is the primary metabolite of **serotonin**. It is the gold-standard urinary marker for **Carcinoid Syndrome**, not pheochromocytoma. * **Option C & D:** Since VMA is specific to catecholamine metabolism and 5-HIAA is specific to serotonin metabolism, they represent two distinct clinical entities. **3. NEET-PG High-Yield Clinical Pearls:** * **Best Initial Screening Test:** Plasma free metanephrines (highest sensitivity). * **Confirmatory Test:** 24-hour urinary metanephrines and VMA (high specificity). * **Rule of 10s:** 10% are bilateral, 10% are malignant, 10% are extra-adrenal (Paragangliomas), and 10% occur in children [1]. * **Clinical Triad:** Episodic headache, sweating (diaphoresis), and tachycardia/palpitations in a hypertensive patient. * **Pre-operative Management:** Always give **Alpha-blockers first** (e.g., Phenoxybenzamine) followed by Beta-blockers to prevent a hypertensive crisis.

Q: A child presents with pellagra-like symptoms and amino acids in the urine. There is a family history of two affected siblings and two unaffected siblings, while the parents are normal. What is the diagnosis?

Hartnup disease. ### Explanation **Correct Option: D. Hartnup disease** **Mechanism:** Hartnup disease is an **autosomal recessive** disorder caused by a mutation in the **SLC6A19 gene**, which encodes a neutral amino acid transporter in the proximal renal tubules and intestinal mucosa. This leads to the malabsorption and excessive urinary loss of neutral amino acids, most notably **Tryptophan**. Tryptophan is a precursor for **Niacin (Vitamin B3)**. A deficiency in Tryptophan results in secondary Niacin deficiency, manifesting as **Pellagra-like symptoms**: Dermatitis (photosensitive rash), Diarrhea, and Dementia/Ataxia. The family history described (affected siblings, normal parents) is classic for an autosomal recessive inheritance pattern. --- ### Why Other Options are Incorrect: * **A. Phenylketonuria (PKU):** Caused by a deficiency of Phenylalanine Hydroxylase. It presents with intellectual disability, "mousy" body odor, and hypopigmentation, but not pellagra-like dermatitis. * **B. Alkaptonuria:** A deficiency of Homogentisate Oxidase. It is characterized by dark urine (on standing), ochronosis (bluish-black pigmentation of connective tissue), and arthritis. * **C. Maple Syrup Urine Disease (MSUD):** A deficiency of the Branched-Chain Alpha-Keto Acid Dehydrogenase complex. It presents in neonates with poor feeding, vomiting, and a characteristic "maple syrup" or burnt sugar odor in the urine. --- ### NEET-PG High-Yield Pearls: * **The "3 Ds" of Pellagra:** Dermatitis, Diarrhea, Dementia (and eventually Death). * **Biochemical Pathway:** Tryptophan → Niacin → NAD+/NADP+. * **Diagnosis:** Confirmed by detecting **neutral aminoaciduria** (specifically Alanine, Serine, Threonine, Valine, Leucine, Isoleucine, Phenylalanine, Tyrosine, and Tryptophan) via paper chromatography. * **Treatment:** High-protein diet and **Nicotinic acid (Niacin) supplementation**.

Q: Which of the following is NOT true about the transamination reaction?

Biotin is required as a coenzyme.. ### Explanation **1. Why Option D is the Correct Answer (The Concept):** Transamination is the first step in the catabolism of most amino acids. The reaction requires **Pyridoxal Phosphate (PLP)**, a derivative of **Vitamin B6**, as a mandatory coenzyme—not Biotin. PLP acts as a temporary carrier for the amino group, forming a Schiff base intermediate (Pyridoxamine phosphate). Biotin, on the other hand, is a coenzyme for **carboxylation** reactions (e.g., Pyruvate carboxylase). **2. Analysis of Incorrect Options:** * **Option A:** This is the fundamental definition of transamination. It involves the reversible transfer of an amino group from an amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. * **Option B:** Alpha-ketoglutarate is indeed the universal acceptor. It accepts the amino group to become **Glutamate**, which then undergoes oxidative deamination in the liver to release ammonia. * **Option C:** While most amino acids participate in transamination, there are notable exceptions. **Threonine, Lysine, Proline, and Hydroxyproline** do not undergo transamination; they are instead deaminated or metabolized via other pathways. **3. NEET-PG High-Yield Clinical Pearls:** * **Enzymes:** Transaminases (Aminotransferases) are located in both the cytosol and mitochondria. * **Diagnostic Markers:** AST (SGOT) and ALT (SGPT) are clinical markers of liver/cardiac damage. ALT is more specific for the liver. * **The "Funneling" Effect:** Transamination "funnels" nitrogen from various amino acids into Glutamate, which is the only amino acid that undergoes rapid oxidative deamination via **Glutamate Dehydrogenase (GDH)**. * **Equilibrium:** Transamination reactions are freely reversible ($K_{eq} \approx 1$).

Q: The presence of a positive Ferric chloride test in urine is attributed to which of the following compounds?

Phenylpyruvate. ### Explanation **Correct Answer: C. Phenylpyruvate** The **Ferric chloride (FeCl₃) test** is a classic biochemical screening tool used to detect specific metabolites in urine. In patients with **Phenylketonuria (PKU)**, a deficiency of the enzyme *Phenylalanine Hydroxylase* leads to the accumulation of Phenylalanine. When the primary metabolic pathway is blocked, Phenylalanine undergoes alternative metabolism via transamination to form **Phenylpyruvate** (a phenylketone), Phenylacetate, and Phenyllactate. The Ferric chloride test specifically reacts with the **enol group of Phenylpyruvate**, producing a characteristic **transient blue-green color**. Therefore, while Phenylalanine is the primary amino acid that increases in the blood, it is its keto-acid derivative, Phenylpyruvate, that is responsible for the positive urine test. **Analysis of Incorrect Options:** * **A. Phenylalanine:** This is the substrate that accumulates in the blood, but it does not react with Ferric chloride to produce a color change. * **B. Phenylacetate:** This metabolite is responsible for the characteristic **"mousy" or "musty" odor** of urine in PKU patients, but it does not give a positive Ferric chloride test. * **D. Phenylaspartate:** This is not a significant metabolite in the phenylalanine degradation pathway. **High-Yield Clinical Pearls for NEET-PG:** * **PKU Presentation:** Intellectual disability, hypopigmentation (fair skin/blue eyes due to decreased melanin), and seizures. * **Guthrie Test:** A bacterial inhibition assay (using *B. subtilis*) used for neonatal screening of PKU; it detects Phenylalanine, not Phenylpyruvate. * **Other FeCl₃ Test Results:** * **Alkaptonuria:** Transient deep blue/green. * **Maple Syrup Urine Disease (MSUD):** Navy blue. * **Tyrosinemia:** Pale green. * **Salicylates/Phenothiazines:** Purple/Violet.

Q: What is the enzyme deficiency in albinism?

Tyrosinase. **Explanation:** **1. Why Tyrosinase is the Correct Answer:** Albinism (specifically Oculocutaneous Albinism Type 1) is caused by a deficiency of the copper-containing enzyme **Tyrosinase**. In the melanocytes, Tyrosinase is the rate-limiting enzyme responsible for the first two steps of melanin synthesis: the hydroxylation of **Tyrosine to DOPA** (Dihydroxyphenylalanine) and the subsequent oxidation of **DOPA to Dopaquinone**. A lack of this enzyme results in a total or partial absence of melanin pigment in the skin, hair, and eyes. **2. Analysis of Incorrect Options:** * **B. Tyrosine hydroxylase:** While it also converts Tyrosine to DOPA, this enzyme is found in the central nervous system and adrenal medulla. It is the rate-limiting step for **catecholamine synthesis** (Dopamine, Epinephrine, Norepinephrine), not melanin. * **C. Phenylalanine hydroxylase:** Deficiency of this enzyme leads to **Phenylketonuria (PKU)**. While PKU patients may have fair skin due to secondary tyrosine deficiency, it is not the primary cause of albinism. * **D. Homogentisate oxidase:** Deficiency of this enzyme leads to **Alkaptonuria**, characterized by the accumulation of homogentisic acid, resulting in dark urine and ochronosis (pigmentation of connective tissue), which is the opposite of albinism. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** Most forms of Albinism are **Autosomal Recessive**. * **Clinical Features:** Photophobia, nystagmus, decreased visual acuity, and a high risk of **Squamous Cell Carcinoma** of the skin. * **Biochemical Link:** Tyrosine is the common precursor for Melanin, Catecholamines, and Thyroid hormones (T3/T4). * **Key Test:** The **Hair bulb incubation test** can help differentiate between tyrosinase-positive and tyrosinase-negative albinism.

Question 1

The Cahill cycle (also known as the Cori cycle) occurs between which two organs?

Accepted Answer

Brain and muscle

Answer

Liver and brain

Answer

Liver and muscle

Answer

Brain only

Question 2

A child presents with microcephaly, blue eyes, fair skin, mental retardation, and a positive ferric chloride test. What is the likely diagnosis?

Accepted Answer

Phenylketonuria (PKU)

Answer

Homocystinuria

Answer

Tyrosinosis

Answer

Alkaptonuria

Question 3

Which of the following is NOT an enzyme of the urea cycle?

Accepted Answer

Citrulline synthase

Answer

Ornithine transcarbamylase

Answer

Carbamoyl-phosphate synthetase I

Answer

Argininosuccinase

Question 4

Diagnosis of carcinoid tumor is done by?

Accepted Answer

5-HIAA

Answer

DHEA

Answer

VMA

Answer

Metanephrines

Question 5

Which amino acid plays a role in the sleep-wake cycle?

Accepted Answer

Tryptophan

Answer

Phenylalanine

Answer

Glycine

Answer

Glutamine

Question 6

In phaeochromocytoma, what substance will be present in elevated amounts in the urine?

Accepted Answer

Vanillylmandelic acid (VMA)

Answer

5-hydroxyindoleacetic acid (HIAA)

Answer

Both VMA and HIAA

Answer

Neither VMA nor HIAA

Question 7

A child presents with pellagra-like symptoms and amino acids in the urine. There is a family history of two affected siblings and two unaffected siblings, while the parents are normal. What is the diagnosis?

Accepted Answer

Hartnup disease

Answer

Phenylketonuria (PKU)

Answer

Alkaptonuria

Answer

Maple syrup urine disease (MSUD)

Question 8

Which of the following is NOT true about the transamination reaction?

Accepted Answer

Biotin is required as a coenzyme.

Answer

It involves the transfer of an alpha-amino group from an alpha-amino acid to a keto acid.

Answer

Alpha-ketoglutarate is the most common alpha-keto acid acceptor.

Answer

Threonine does not undergo transamination.

Question 9

The presence of a positive Ferric chloride test in urine is attributed to which of the following compounds?

Accepted Answer

Phenylpyruvate

Answer

Phenylalanine

Answer

Phenylacetate

Answer

Phenylaspartate

Question 10

What is the enzyme deficiency in albinism?

Accepted Answer

Tyrosinase

Answer

Tyrosine hydroxylase

Answer

Phenylalanine hydroxylase

Answer

Homogentisate oxidase

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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