Amino Acid Metabolism Practice Questions

Q: All of the following are ketogenic and glucogenic amino acids except?

Serine. ### Explanation The classification of amino acids based on their metabolic fate is a high-yield topic for NEET-PG. Amino acids are categorized into three groups based on whether their carbon skeletons can be converted into glucose (glucogenic), ketone bodies (ketogenic), or both. **1. Why Serine is the Correct Answer:** **Serine** is a **purely glucogenic** amino acid. Its carbon skeleton is converted into **pyruvate**, which can then enter the gluconeogenesis pathway to produce glucose. It cannot be converted into Acetyl-CoA or Acetoacetate (the precursors for ketone bodies); therefore, it lacks ketogenic properties. **2. Analysis of Incorrect Options (Both Glucogenic & Ketogenic):** These amino acids are "amphibolic" because their breakdown products include both glucose precursors (like Succinyl-CoA or Fumarate) and ketone precursors (like Acetyl-CoA). * **Tyrosine & Phenylalanine (Options A & C):** These aromatic amino acids are metabolized into **Fumarate** (glucogenic) and **Acetoacetate** (ketogenic). * **Isoleucine (Option B):** This branched-chain amino acid is degraded into **Succinyl-CoA** (glucogenic) and **Acetyl-CoA** (ketogenic). * *Note:* Tryptophan is the fourth major amino acid that is both glucogenic and ketogenic. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Purely Ketogenic:** Only two amino acids are purely ketogenic—**Leucine and Lysine** (Mnemonic: The "L"s are purely ketogenic). * **Both Glucogenic & Ketogenic:** Remember the mnemonic **"PITTT"**: **P**henylalanine, **I**soleucine, **T**yrosine, **T**ryptophan, and **T**hreonine. * **Purely Glucogenic:** All remaining 13 amino acids (including Serine, Glycine, Valine, etc.). * **Clinical Correlation:** In ketogenic diets or states of starvation, the body relies on ketogenic amino acids to produce ketone bodies (acetoacetate and β-hydroxybutyrate) as an alternative fuel source for the brain.

Q: Deficiency of arginosuccinate synthase causes which of the following disorders?

Citrullinemia. **Explanation:** The Urea Cycle is the primary pathway for disposing of nitrogenous waste. A deficiency in any of its five core enzymes leads to a specific Urea Cycle Disorder (UCD). **1. Why Citrullinemia is Correct:** **Argininosuccinate synthase (ASS)** is the third enzyme of the urea cycle. It catalyzes the condensation of **citrulline** and **aspartate** to form argininosuccinate. When ASS is deficient (Type I Citrullinemia), citrulline cannot be converted and subsequently accumulates in the blood and urine. This block also leads to a backup of ammonia, causing hyperammonemia. **2. Analysis of Incorrect Options:** * **Hyperargininemia:** Caused by a deficiency of **Arginase**, the final enzyme of the cycle. It is unique because it often presents with spastic diplegia rather than acute neonatal hyperammonemia. * **Argininosuccinic aciduria:** Caused by a deficiency of **Argininosuccinate lyase (ASL)**. This results in the accumulation of argininosuccinate. A classic clinical sign is *trichorrhexis nodosa* (fragile, knotted hair). * **Type I Hyperammonemia:** Caused by a deficiency of **Carbamoyl phosphate synthetase I (CPS-I)**, the rate-limiting step. It is the most severe form of urea cycle disorder. (Note: Type II is caused by OTC deficiency). **High-Yield NEET-PG Pearls:** * **Most Common UCD:** Ornithine Transcarbamoylase (OTC) deficiency (the only X-linked UCD; all others are Autosomal Recessive). * **Orotic Aciduria:** Present in OTC deficiency but **absent** in CPS-I deficiency. * **Management:** Acute treatment involves ammonia scavengers like **Sodium benzoate** or **Sodium phenylbutyrate**, which provide alternative pathways for nitrogen excretion.

Q: All of the following are characteristics of phenylalanine hydroxylase, EXCEPT?

Vitamin C is a cofactor. **Explanation:** The conversion of Phenylalanine to Tyrosine is catalyzed by **Phenylalanine Hydroxylase (PAH)**. This reaction is the rate-limiting step in phenylalanine catabolism. **Why Option D is the Correct Answer (The Exception):** **Vitamin C (Ascorbic acid)** is not a cofactor for Phenylalanine Hydroxylase. Vitamin C is, however, a required cofactor for **p-hydroxyphenylpyruvate hydroxylase** (the next step in tyrosine catabolism) and **Dopamine β-hydroxylase**. Its absence leads to Scurvy, not Phenylketonuria (PKU). **Analysis of Incorrect Options:** * **Option A (Mixed function oxidase):** PAH is classified as a monooxygenase or mixed-function oxidase because it incorporates one atom of molecular oxygen ($O_2$) into the substrate (forming the -OH group of Tyrosine) while the other oxygen atom is reduced to water ($H_2O$). * **Option B (Tetrahydrobiopterin):** **$BH_4$** is the essential coenzyme for PAH. It donates electrons to reduce oxygen and is oxidized to Dihydrobiopterin ($BH_2$) during the reaction. * **Option C (NADPH):** While NADPH does not participate directly in the hydroxylation, it is essential for the **regeneration of $BH_4$** from $BH_2$ via the enzyme Dihydrobiopterin reductase. Thus, NADPH provides the ultimate reducing power for the cycle to continue. **High-Yield Clinical Pearls for NEET-PG:** * **Phenylketonuria (PKU):** Most commonly caused by a deficiency of PAH (Classical PKU). A rare "Malignant" form is caused by a deficiency of Dihydrobiopterin reductase. * **Diagnostic Marker:** "Mousy" or "Musty" body odor due to phenylacetic acid in sweat and urine. * **Tyrosine:** Becomes an **essential amino acid** in patients with PKU. * **Screening:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry.

Q: "Marfan-like syndrome" is associated with which of the following conditions?

Homocystinuria. **Explanation:** **Homocystinuria** is the correct answer because it is a metabolic disorder characterized by an accumulation of homocysteine, which interferes with the cross-linking of collagen and elastin fibers. This biochemical defect leads to a phenotype known as **"Marfanoid habitus"** (tall stature, long extremities, and arachnodactyly). The most common cause is a deficiency of the enzyme **Cystathionine β-synthase (CBS)**. While it shares skeletal features with Marfan syndrome, a key clinical differentiator is the direction of **lens subluxation (ectopia lentis)**: in Homocystinuria, the lens typically displaces **downward and inward** (inferomedial), whereas in Marfan syndrome, it displaces upward and outward (superolateral). **Why other options are incorrect:** * **Alkaptonuria:** Caused by a deficiency of homogentisate oxidase. It presents with ochronosis (dark pigmentation of connective tissues), dark urine upon standing, and arthritis, but not Marfan-like features. * **Phenylketonuria (PKU):** Results from phenylalanine hydroxylase deficiency. Clinical hallmarks include intellectual disability, "mousy" body odor, and hypopigmentation (fair skin/blue eyes). * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-Chain Alpha-Keto Acid Dehydrogenase complex. It presents with a "maple syrup" odor in urine and severe neurological deterioration in infancy. **NEET-PG High-Yield Pearls:** * **Inheritance:** Homocystinuria is Autosomal Recessive (Marfan is Autosomal Dominant). * **Vascular Risk:** Patients have a high risk of **thromboembolism** and premature atherosclerosis. * **Treatment:** A subset of patients responds to high doses of **Vitamin B6 (Pyridoxine)**, which is a cofactor for CBS. * **Intellectual Disability:** Common in Homocystinuria, but absent in classic Marfan syndrome.

Q: What is the most direct precursor of taurine?

Cysteine. **Explanation:** **Taurine** is a sulfur-containing amino sulfonic acid that plays a critical role in bile acid conjugation (forming taurocholic acid) and neurotransmission. **1. Why Cysteine is correct:** The synthesis of taurine occurs primarily in the liver via the **cysteine sulfinic acid pathway**. * **Step 1:** Cysteine is oxidized to **Cysteine sulfinate** by the enzyme *cysteine dioxygenase*. * **Step 2:** Cysteine sulfinate undergoes decarboxylation (requiring Vitamin B6/PLP) to form **Hypotaurine**. * **Step 3:** Hypotaurine is oxidized to form **Taurine**. Since taurine is derived directly from the oxidation and decarboxylation of cysteine, cysteine is its immediate and most direct precursor. **2. Why other options are incorrect:** * **Methionine:** While methionine is the ultimate source of sulfur for taurine, it must first be converted to homocysteine and then to cysteine via the transsulfuration pathway. It is a precursor, but not the *direct* one. * **Glycine:** Glycine is used for the conjugation of bile acids (forming glycocholic acid), but it does not serve as a structural precursor for taurine. * **Glutathione:** This is a tripeptide (Glu-Cys-Gly) that serves as a major antioxidant. While it contains cysteine, it is a downstream product/storage form rather than a direct precursor in the taurine biosynthetic pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Bile Acid Conjugation:** Taurine and Glycine are the two amino acids used to conjugate bile acids, making them more water-soluble for excretion into the intestine. * **Cofactor Requirement:** The decarboxylation step in taurine synthesis requires **Pyridoxal Phosphate (Vitamin B6)**. * **Essentiality:** Taurine is considered a "conditionally essential" amino acid, especially in neonates who have limited capacity for its synthesis.

Q: A patient presents with recurrent kidney stones. Microscopic examination of the urine specimen is shown below. Which of the following is not seen in the urine of this patient?

Cysteine. ***Cysteine*** - **Cysteine** is the free, reduced monomer form that is **not found in urine** during cystinuria as it is readily reabsorbed by normal transporters. - The condition involves defective transport of **cystine** (the oxidized disulfide dimer) and dibasic amino acids, not the free cysteine monomer. *Lysine* - **Lysine** is one of the **dibasic amino acids** affected in cystinuria and is found in increased amounts in urine. - It is part of the **COAL mnemonic** (Cystine, Ornithine, Arginine, Lysine) representing amino acids with defective renal tubular transport. *Arginine* - **Arginine** is another **dibasic amino acid** that accumulates in urine due to defective renal tubular reabsorption. - Along with lysine, it contributes to the **alkaline pH** that promotes cystine stone formation in the urinary tract. *Cystine* - **Cystine** is the oxidized disulfide dimer that forms **hexagonal crystals** visible on urine microscopy in cystinuria patients. - It is poorly soluble in acidic urine and readily precipitates to form **recurrent kidney stones** characteristic of this genetic disorder.

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).

Question 1

All of the following are ketogenic and glucogenic amino acids except?

Accepted Answer

Serine

Answer

Tyrosine

Answer

Isoleucine

Answer

Phenylalanine

Question 2

Deficiency of arginosuccinate synthase causes which of the following disorders?

Accepted Answer

Citrullinemia

Answer

Hyperargininemia

Answer

Argininosuccinic aciduria

Answer

Type I hyperammonemia

Question 3

All of the following are characteristics of phenylalanine hydroxylase, EXCEPT?

Accepted Answer

Vitamin C is a cofactor

Answer

Mixed function oxidase

Answer

Tetrahydrobiopterin is a cofactor

Answer

NADPH provides the reducing power

Question 4

"Marfan-like syndrome" is associated with which of the following conditions?

Accepted Answer

Homocystinuria

Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Maple syrup disease

Question 5

Which enzyme of the urea cycle is deficient in the brain?

Accepted Answer

Ornithine Transcarbamoylase

Answer

Arginase

Answer

Carbamoyl phosphate synthetase I

Answer

Argininosuccinate Lyase

Question 6

What is the most direct precursor of taurine?

Accepted Answer

Cysteine

Answer

Glycine

Answer

Methionine

Answer

Glutathione

Question 7

A patient presents with recurrent kidney stones. Microscopic examination of the urine specimen is shown below. Which of the following is not seen in the urine of this patient?

Accepted Answer

Cysteine

Answer

Lysine

Answer

Arginine

Answer

Cystine

Question 8

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 9

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 10

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

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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