Amino Acid Metabolism Practice Questions

Q: Which of the following represents the nontoxic form for storage and transportation of ammonia?

Glutamine. **Explanation:** **Why Glutamine is the Correct Answer:** Ammonia ($NH_3$) is highly toxic, especially to the central nervous system. To prevent toxicity during transport from peripheral tissues (like the brain and muscles) to the liver, ammonia is converted into **Glutamine**. The enzyme **Glutamine Synthetase** catalyzes the addition of an ammonia molecule to Glutamate, requiring ATP. Glutamine is a neutral, non-toxic amino acid that can easily traverse cell membranes. Once it reaches the liver or kidneys, the enzyme **Glutaminase** removes the ammonia, allowing it to enter the Urea Cycle for excretion. Thus, Glutamine serves as the primary "nontoxic carrier" and storage form of ammonia in the blood. **Why the Other Options are Incorrect:** * **Glutamate/Glutamic acid (Options B & D):** While Glutamate is the immediate precursor to Glutamine, it is an acidic amino acid and a major excitatory neurotransmitter. High levels of intracellular glutamate can lead to excitotoxicity; therefore, it is not the preferred transport form. * **Aspartic acid (Option A):** Aspartic acid plays a role in the Urea Cycle by providing the second nitrogen atom (reacting with Citrulline to form Argininosuccinate), but it is not the primary systemic transporter of free ammonia. **High-Yield Clinical Pearls for NEET-PG:** * **Glucose-Alanine Cycle:** While Glutamine is the universal transporter, **Alanine** is the specific transport form of ammonia from **Skeletal Muscle** to the liver. * **Brain Protection:** The conversion of ammonia to glutamine is the brain's primary mechanism for detoxification. However, excessive glutamine accumulation in astrocytes leads to osmotic swelling, contributing to **Hepatic Encephalopathy**. * **Rate-limiting step:** The Urea Cycle is the definitive way the body disposes of ammonia, with **CPS-I** being the rate-limiting enzyme.

Q: In Hartnup disease, which amino acid is considered the limiting amino acid?

Tryptophan. ### Explanation **1. Why Tryptophan is the Correct Answer:** Hartnup disease is an autosomal recessive disorder caused by a mutation in the **SLC6A19 gene**, which encodes a sodium-dependent neutral amino acid transporter (System B0) located in the proximal renal tubules and the intestinal mucosa. This defect leads to the malabsorption of neutral amino acids, most significantly **Tryptophan**. Tryptophan is considered the "limiting" amino acid because it is the essential precursor for the synthesis of **Niacin (Vitamin B3)** via the kynurenine pathway. In Hartnup disease, the deficiency of Tryptophan leads to a secondary deficiency of Niacin, resulting in **pellagra-like symptoms** (Dermatitis, Diarrhea, Dementia). **2. Why the Other Options are Incorrect:** * **Option A (Tyrosine):** While Tyrosine is a neutral amino acid, its deficiency does not drive the primary clinical pathology of Hartnup disease. Tyrosine is primarily associated with Alkaptonuria or Tyrosinemia. * **Option C (Phenylalanine):** Phenylalanine is also a neutral amino acid affected by the transporter defect, but its clinical impact is negligible compared to Tryptophan. Phenylalanine metabolism is primarily discussed in the context of Phenylketonuria (PKU). **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Characterized by a photosensitive scaly rash (pellagra-like), intermittent cerebellar ataxia, and aminoaciduria. * **Diagnostic Hallmark:** **Neutral Aminoaciduria** (presence of neutral amino acids in urine, but *not* proline, hydroxyproline, or arginine). * **The "Indican" Connection:** Unabsorbed Tryptophan in the gut is converted by bacteria into indoles, which are absorbed and excreted in the urine as **Indican** (Blue diaper syndrome is a related differential). * **Treatment:** High-protein diet and **Nicotinic acid (Niacin) supplementation**.

Q: An infant presents with a history of vomiting and poor feeding. A musty odor is noted in the baby's urine. A Guthrie test was performed and found to be positive. What is the most likely diagnosis?

Phenylketonuria. **Explanation:** **1. Why Phenylketonuria (PKU) is the correct answer:** The clinical triad of **vomiting/poor feeding**, a characteristic **"musty" or "mousy" odor** in the urine, and a **positive Guthrie test** is pathognomonic for Phenylketonuria. PKU is most commonly caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**, which converts Phenylalanine to Tyrosine. The accumulation of phenylalanine leads to the production of alternative metabolites like phenylacetate (responsible for the odor). The Guthrie test is a bacterial inhibition assay where *B. subtilis* growth occurs only in the presence of high phenylalanine levels, confirming the diagnosis. **2. Why the other options are incorrect:** * **Alkaptonuria:** Characterized by a deficiency of Homogentisate oxidase. Clinical features include urine that turns **black on standing** and ochronosis (pigmentation of connective tissue), not a musty odor or positive Guthrie test. * **Tyrosinemia:** Presents with severe liver involvement (jaundice, hepatomegaly) and a characteristic **"cabbage-like" odor**. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-chain alpha-keto acid dehydrogenase complex. It presents with a **"burnt sugar" or "maple syrup" odor** in the urine. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Cofactor:** A small percentage of cases are due to a deficiency in **Dihydrobiopterin reductase (BH4)**. * **Clinical Feature:** Patients often have fair skin and blue eyes due to decreased melanin synthesis (Tyrosine is a precursor to melanin). * **Management:** Dietary restriction of Phenylalanine and supplementation of Tyrosine (which becomes an essential amino acid in PKU).

Q: Which of the following is NOT true about phenylketonuria?

Coke coloured urine. **Explanation:** **Phenylketonuria (PKU)** is an autosomal recessive inborn error of metabolism. The correct answer is **D (Coke-coloured urine)** because this clinical finding is characteristic of **Alkaptonuria** (due to homogentisic acid oxidase deficiency), not PKU. 1. **Why Option D is correct (The False Statement):** In PKU, the urine typically has a characteristic **"mousy" or "musty" odor** due to the presence of phenylacetic acid. Coke-colored or dark urine that turns black upon standing is the hallmark of Alkaptonuria, where homogentisic acid undergoes oxidation. 2. **Why Option A is incorrect:** PKU most commonly occurs due to a deficiency of the enzyme **phenylalanine hydroxylase (PAH)**, which converts phenylalanine to tyrosine. (A rarer form involves a deficiency of the cofactor Tetrahydrobiopterin, BH4). 3. **Why Option B is incorrect:** When the primary pathway is blocked, phenylalanine is diverted to alternative pathways, producing **phenylpyruvate**, phenyllactate, and phenylacetate. Phenylpyruvate is a keto-acid excreted in high amounts in the urine. 4. **Why Option C is incorrect:** Untreated PKU leads to severe intellectual disability, developmental delays (failure to walk/talk), and neurological symptoms like seizures and tremors due to the neurotoxic effects of high phenylalanine levels and decreased neurotransmitter synthesis. **NEET-PG High-Yield Pearls:** * **Guthrie Test:** A bacterial inhibition assay used for neonatal screening of PKU. * **Ferric Chloride Test:** Gives a **transient blue-green** color in PKU (compared to the deep green in Alkaptonuria). * **Clinical Triad:** Intellectual disability, mousy odor, and **hypopigmentation** (fair skin/blue eyes) due to decreased melanin synthesis from tyrosine. * **Management:** Dietary restriction of phenylalanine and supplementation of tyrosine (which becomes an essential amino acid).

Q: Urea is formed from which substrate?

Aspartate. The urea cycle (Krebs-Henseleit cycle) is the primary mechanism for detoxifying ammonia into urea in the liver. Urea contains two nitrogen atoms: one is derived from **free ammonia** (via carbamoyl phosphate) and the second is derived from **Aspartate**. ### Why Aspartate is the Correct Answer In the third step of the urea cycle, **Citrulline** condenses with **Aspartate** to form **Argininosuccinate**, a reaction catalyzed by *argininosuccinate synthetase*. During this process, the amino group of Aspartate is incorporated into the molecular structure that eventually becomes urea. Therefore, Aspartate is a direct substrate providing the second nitrogen atom of the urea molecule. ### Why Other Options are Incorrect * **Arginine:** While urea is released from Arginine by the enzyme *arginase*, Arginine is considered an intermediate of the cycle rather than the substrate providing the nitrogen atoms. * **Ornithine:** This acts as a "catalytic" carrier. It combines with carbamoyl phosphate to start the cycle and is regenerated at the end. It does not contribute atoms to the urea molecule itself. * **Citrulline:** This is an intermediate formed in the mitochondria that must be transported to the cytosol to react with Aspartate. ### NEET-PG High-Yield Pearls * **Rate-limiting enzyme:** Carbamoyl Phosphate Synthetase I (CPS-I), which requires **N-acetylglutamate (NAG)** as an essential activator. * **Location:** The cycle occurs in both the **Mitochondria** (first two steps) and the **Cytosol** (remaining steps). * **Fumarate Link:** The "Bicycle" or "Krebs-Henseleit" link occurs when Argininosuccinate is cleaved into Arginine and Fumarate; the latter enters the TCA cycle. * **Hyperammonemia Type II:** The most common urea cycle disorder, caused by **Ornithine Transcarbamoylase (OTC) deficiency** (X-linked recessive).

Q: Which amino acid enters the TCA cycle for gluconeogenesis and is ketogenic in nature?

Phenylalanine. ### **Explanation** **1. Why Phenylalanine is Correct:** Amino acids are classified based on their metabolic end-products. **Phenylalanine** is both **glucogenic and ketogenic**. * **Ketogenic component:** It is metabolized into **Acetoacetate**, which can form ketone bodies. * **Glucogenic component:** It is metabolized into **Fumarate**, an intermediate of the TCA cycle. Fumarate can be converted to oxaloacetate, which enters the gluconeogenesis pathway to produce glucose. Other amino acids sharing this dual property include Tyrosine, Tryptophan, and Isoleucine (Mnemonic: **PITTT** – Phenylalanine, Isoleucine, Tyrosine, Tryptophan, Threonine). **2. Why Other Options are Incorrect:** * **Alanine (B):** It is a **purely glucogenic** amino acid. It is converted to **Pyruvate** via transamination (ALT). It is the primary amino acid used by the liver for gluconeogenesis (Glucose-Alanine cycle). * **Glycine (C):** It is **purely glucogenic**. It enters the metabolic pathway as Pyruvate or Glyoxylate. * **Serine (D):** It is **purely glucogenic**. It is converted directly to **Pyruvate** by the enzyme Serine dehydratase. **3. Clinical Pearls & High-Yield Facts:** * **Purely Ketogenic:** Only **Leucine and Lysine** are strictly ketogenic (they do not produce glucose). * **Essentiality:** Phenylalanine is an essential amino acid and the precursor to Tyrosine. * **Clinical Correlation:** A deficiency in **Phenylalanine Hydroxylase** leads to **Phenylketonuria (PKU)**, characterized by intellectual disability and a "mousy" body odor due to phenylacetate accumulation. * **Metabolic Entry:** Remember that Phenylalanine and Tyrosine enter the TCA cycle specifically at the level of **Fumarate**.

Q: The nitroprusside test is used for the diagnosis of which of the following diseases?

Homocystinuria. **Explanation:** The **Cyanide-Nitroprusside test** is a screening tool used to detect the presence of sulfur-containing amino acids in the urine, specifically **Cystine** and **Homocysteine**. In **Homocystinuria**, there is a deficiency of the enzyme *Cystathionine beta-synthase (CBS)*, leading to an accumulation of homocysteine. When sodium cyanide is added to the urine, it reduces the disulfide bonds of homocystine into sulfhydryl groups (homocysteine). These groups then react with sodium nitroprusside to produce a characteristic **magenta/deep red color**, confirming a positive result. **Analysis of Incorrect Options:** * **A. Phenylketonuria (PKU):** Diagnosed using the **Ferric Chloride test**, which produces a blue-green color due to phenylpyruvic acid. * **B. Tyrosinemia:** Also detected by the Ferric Chloride test (transient green) or the **Millon’s test** (detects tyrosine metabolites). * **C. Maple Syrup Urine Disease (MSUD):** Diagnosed using the **DNPH (2,4-Dinitrophenylhydrazine) test**, which detects alpha-keto acids, resulting in a yellow-white precipitate. **High-Yield Clinical Pearls for NEET-PG:** * **Differential Diagnosis:** Both Homocystinuria and **Cystinuria** give a positive Nitroprusside test. To differentiate, remember that Homocystinuria presents with intellectual disability and downward lens subluxation (*Ectopia lentis*), whereas Cystinuria presents with renal stones. * **Treatment Tip:** About 50% of Homocystinuria cases are **Pyridoxine (Vitamin B6) responsive**. * **Vascular Risk:** Homocystinuria is a high-yield cause of premature atherosclerosis and thromboembolism in young patients.

Q: What condition is characterized by a 'boiled cabbage' smell in the urine?

Hypermethioninemia. **Explanation:** The characteristic **'boiled cabbage'** odor in urine is a classic clinical sign of **Hypermethioninemia**. This condition occurs due to a deficiency in the enzyme **Methionine Adenosyltransferase (MAT)**, leading to elevated levels of methionine in the blood and urine. The distinct odor is attributed to the accumulation of methionine metabolites, specifically **alpha-keto-gamma-methiolbutyrate**. **Analysis of Options:** * **Hypermethioninemia (Correct):** Associated with a boiled cabbage or rancid butter smell. It is often benign but can be seen in severe liver disease or Tyrosinemia Type I (which also presents with this odor due to methionine involvement). * **Phenylketonuria (PKU):** Characterized by a **'mousy' or 'musty'** odor due to the accumulation of phenylacetic acid. * **Hawkinsinuria:** A rare defect in tyrosine metabolism characterized by a **'swimming pool' or chlorine-like** odor. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-Chain Alpha-Keto Acid Dehydrogenase complex, leading to a **'burnt sugar' or maple syrup** smell. **NEET-PG High-Yield Pearls (Odors in Inborn Errors of Metabolism):** * **Isovaleric Acidemia:** Sweaty feet / Cheesy odor. * **Trimethylaminuria:** Fishy odor. * **Tyrosinemia Type I:** Boiled cabbage / Rancid butter. * **Multiple Carboxylase Deficiency:** Tomcat urine. * **Oasthouse Urine Disease:** Dried malt / Hops odor.

Q: In which condition is vanillylmandelic acid excreted in urine?

Pheochromocytoma. **Explanation:** **Pheochromocytoma** is a catecholamine-secreting tumor, typically arising from the chromaffin cells of the adrenal medulla. These tumors produce excessive amounts of epinephrine and norepinephrine. In the body, these catecholamines are metabolized by enzymes **Monoamine Oxidase (MAO)** and **Catechol-O-methyltransferase (COMT)**. The final end-product of this metabolic pathway is **Vanillylmandelic Acid (VMA)**. Consequently, elevated 24-hour urinary VMA levels serve as a classic diagnostic marker for Pheochromocytoma. **Analysis of Incorrect Options:** * **Gaucher’s disease:** A lysosomal storage disorder caused by a deficiency of **glucocerebrosidase**, leading to the accumulation of glucocerebroside. It is characterized by hepatosplenomegaly and "wrinkled tissue paper" appearance of macrophages. * **Phenylketonuria (PKU):** Caused by a deficiency of **phenylalanine hydroxylase**. This leads to the excretion of phenylpyruvate, phenyllactate, and phenylacetate (giving a "mousy" odor to urine), not VMA. * **Hartnup’s disease:** A defect in the transport of **neutral amino acids** (like Tryptophan) in the gut and kidneys. It results in pellagra-like symptoms due to niacin deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Rule of 10s:** Pheochromocytoma is 10% bilateral, 10% extra-adrenal, 10% malignant, and 10% familial. * **Metanephrines:** While VMA is a common marker, **urinary and plasma metanephrines** are now considered more sensitive screening tests. * **Associated Syndromes:** Always screen for MEN 2A and 2B, von Hippel-Lindau (VHL) disease, and Neurofibromatosis type 1 in these patients.

Q: Which of the following is an amino acid that can carry out one-carbon substitution during metabolism?

Folic acid. **Explanation:** The correct answer is **Folic acid**. In the context of biochemistry and amino acid metabolism, "one-carbon metabolism" refers to the transfer of single carbon units (such as methyl, methylene, or formyl groups) essential for the synthesis of DNA, RNA, and certain amino acids. **Why Folic Acid is Correct:** Folic acid (Vitamin B9) is the precursor to **Tetrahydrofolate (THF)**, the primary coenzyme responsible for carrying and transferring one-carbon units. THF accepts carbon groups from donors like **Serine, Glycine, and Histidine**. These units are then utilized in critical reactions, such as the conversion of homocysteine to methionine and the synthesis of purines and thymidylate (dTMP). **Analysis of Incorrect Options:** * **Butyric acid:** This is a four-carbon short-chain fatty acid produced by gut microbiota fermentation. It serves as a primary energy source for colonocytes but does not participate in one-carbon transfer. * **Vitamin B:** This is a generic term for a group of vitamins. While specific B-vitamins like B12 (Cobalamin) and B6 (Pyridoxine) are co-factors in these pathways, "Vitamin B" is too non-specific. Folic acid is the definitive carrier of the carbon unit itself. **NEET-PG High-Yield Pearls:** * **Major Carbon Donor:** Serine is the most important source of one-carbon units (converting to Glycine via *Serine Hydroxymethyltransferase*). * **The "Methyl Trap":** A deficiency in Vitamin B12 leads to folate being "trapped" as N5-methyl THF, causing a functional folate deficiency and megaloblastic anemia. * **Clinical Link:** Methotrexate, a common chemotherapeutic agent, acts by inhibiting *Dihydrofolate Reductase (DHFR)*, preventing the regeneration of THF and halting DNA synthesis.

Question 1

Which of the following represents the nontoxic form for storage and transportation of ammonia?

Accepted Answer

Glutamine

Answer

Aspartic acid

Answer

Glutamate

Answer

Glutamic acid

Question 2

In Hartnup disease, which amino acid is considered the limiting amino acid?

Accepted Answer

Tryptophan

Answer

Tyrosine

Answer

Phenylalanine

Answer

None of the above

Question 3

An infant presents with a history of vomiting and poor feeding. A musty odor is noted in the baby's urine. A Guthrie test was performed and found to be positive. What is the most likely diagnosis?

Accepted Answer

Phenylketonuria

Answer

Alkaptonuria

Answer

Tyrosinemia

Answer

Maple syrup urine disease

Question 4

Which of the following is NOT true about phenylketonuria?

Accepted Answer

Coke coloured urine

Answer

Occurs due to deficiency of phenylalanine hydroxylase

Answer

High amount of phenylpyruvate in urine

Answer

Failure to talk or walk, seizures and tremors

Question 5

Urea is formed from which substrate?

Accepted Answer

Aspartate

Answer

Arginine

Answer

Ornithine

Answer

Citrulline

Question 6

Which amino acid enters the TCA cycle for gluconeogenesis and is ketogenic in nature?

Accepted Answer

Phenylalanine

Answer

Alanine

Answer

Glycine

Answer

Serine

Question 7

The nitroprusside test is used for the diagnosis of which of the following diseases?

Accepted Answer

Homocystinuria

Answer

Phenylketonuria

Answer

Tyrosinemia

Answer

Maple syrup urine disease

Question 8

What condition is characterized by a 'boiled cabbage' smell in the urine?

Accepted Answer

Hypermethioninemia

Answer

Phenylketonuria

Answer

Hawkinsinuria

Answer

Maple Syrup Urine Disease

Question 9

In which condition is vanillylmandelic acid excreted in urine?

Accepted Answer

Pheochromocytoma

Answer

Gaucher's disease

Answer

Phenylketonuria

Answer

Hartnup's disease

Question 10

Which of the following is an amino acid that can carry out one-carbon substitution during metabolism?

Accepted Answer

Folic acid

Answer

Butyric acid

Answer

Vitamin B

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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