Amino Acid Metabolism Practice Questions

Q: OTC deficiency causes which of the following conditions?

Hyperammonemia type-2. **Explanation:** **Ornithine Transcarbamoylase (OTC) deficiency** is the most common urea cycle disorder. It is unique because it is the only urea cycle defect inherited as an **X-linked recessive** trait; all others are autosomal recessive. 1. **Why Hyperammonemia Type-2 is correct:** The urea cycle begins in the mitochondria. The second step involves the enzyme **OTC**, which catalyzes the reaction between Carbamoyl Phosphate and Ornithine to form Citrulline. A deficiency in OTC leads to an accumulation of Carbamoyl Phosphate and Ammonia, resulting in **Hyperammonemia Type-2**. Excess Carbamoyl Phosphate leaks into the cytosol and enters the pyrimidine synthesis pathway, leading to increased production of **Orotic acid** (a key diagnostic marker). 2. **Why other options are incorrect:** * **Hyperammonemia Type-1:** Caused by a deficiency of **Carbamoyl Phosphate Synthetase I (CPS-I)**. Unlike OTC deficiency, there is no orotic aciduria here. * **Citrullinemia:** Caused by a deficiency of **Argininosuccinate Synthetase**, leading to a massive buildup of Citrulline. * **Argininemia:** Caused by a deficiency of **Arginase**, the final enzyme of the cycle. It typically presents with spastic diplegia rather than acute neonatal hyperammonemia. **High-Yield Clinical Pearls for NEET-PG:** * **Orotic Aciduria:** Differentiates OTC deficiency (High orotic acid) from CPS-I deficiency (Low/Normal orotic acid). * **Management:** Treatment involves a low-protein diet and ammonia-scavenging drugs like **Sodium Benzoate** or **Phenylbutyrate**. * **Citrate/Arginine:** Citrulline levels are typically low in OTC deficiency.

Q: Calcification of intervertebral discs is characteristic of which condition?

Alkaptonuria. ### Explanation **Correct Answer: A. Alkaptonuria** Alkaptonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate 1,2-dioxygenase**, leading to the accumulation of **Homogentisic Acid (HGA)**. The underlying pathology involves the oxidation of HGA into a melanin-like polymer called **alkapton**. This pigment deposits in connective tissues, a process known as **Ochronosis**. In the musculoskeletal system, these deposits weaken the collagen matrix of the intervertebral discs, leading to premature degeneration and **dystrophic calcification**. On X-ray, this presents as a classic "wafer-like" calcification of multiple intervertebral discs, often resulting in "bamboo spine" appearance similar to ankylosing spondylitis. **Why Incorrect Options are Wrong:** * **B. Phenylketonuria:** Caused by Phenylalanine Hydroxylase deficiency. It presents with intellectual disability, "mousy" odor, and hypopigmentation, but does not involve disc calcification. * **C. Gout:** Caused by monosodium urate crystal deposition. While it affects joints (Podagra), it typically involves peripheral joints and causes "punched-out" erosions rather than disc calcification. * **D. Rickets:** A defect in bone mineralization (Vitamin D deficiency). It leads to softening of bones (osteomalacia) and widening of growth plates, not pathological calcification of discs. **Clinical Pearls for NEET-PG:** * **Triad of Alkaptonuria:** 1. Homogentisic aciduria (urine turns black on standing/alkalinization), 2. Ochronosis (blue-black pigmentation of sclera and ear cartilage), 3. Ochronotic arthritis (large joints and spine). * **Diagnostic Test:** Ferric Chloride test (turns deep blue/green) and Silver Nitrate test. * **Management:** High doses of Vitamin C (antioxidant) and **Nitisinone** (inhibits HGA production).

Q: A patient with Gyrate Atrophy due to defective ornithine aminotransferase would be benefited by which dietary modification?

Arginine free diet. ### Explanation **Gyrate Atrophy** is an autosomal recessive dystrophy caused by a deficiency in the mitochondrial enzyme **Ornithine Aminotransferase (OAT)**. This enzyme is responsible for converting ornithine into glutamate or proline. #### 1. Why "Arginine-free diet" is correct: In OAT deficiency, ornithine cannot be metabolized, leading to **hyperornithinemia** (10–20 fold increase). Since **Arginine** is the direct metabolic precursor of ornithine in the urea cycle, restricting dietary arginine intake is the most effective way to lower systemic ornithine levels. Reducing ornithine levels can slow the progression of the characteristic "punched-out" chorioretinal atrophy. #### 2. Analysis of Incorrect Options: * **Option A (Ornithine-free diet):** Ornithine is not typically consumed directly in significant quantities from the diet; it is synthesized endogenously from arginine. Therefore, restricting the precursor (arginine) is the clinical standard. * **Option C & D (Vitamin Supplementation):** While **Pyridoxine (Vitamin B6)** is a cofactor for the OAT enzyme and a small subset of patients are "B6-responsive," Vitamin B1 and B12 have no therapeutic role in this specific condition. #### 3. High-Yield Clinical Pearls for NEET-PG: * **Clinical Presentation:** Patients present with night blindness (nyctalopia) and constricted visual fields in the first decade of life. * **Fundus Findings:** Characteristic well-defined, circular (gyrate) patches of chorioretinal atrophy starting in the mid-periphery and spreading centrally. * **Associated Findings:** Posterior subcapsular cataract and high myopia are common. * **Management Summary:** 1. Arginine-restricted diet. 2. Pyridoxine (B6) supplementation (to enhance residual enzyme activity). 3. Lysine supplementation (to compete with ornithine transport).

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.

Question 1

OTC deficiency causes which of the following conditions?

Accepted Answer

Hyperammonemia type-2

Answer

Citrullinemia

Answer

Argininemia

Answer

Hyperammonemia type-1

Question 2

Calcification of intervertebral discs is characteristic of which condition?

Accepted Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Gout

Answer

Rickets

Question 3

A patient with Gyrate Atrophy due to defective ornithine aminotransferase would be benefited by which dietary modification?

Accepted Answer

Arginine free diet

Answer

Ornithine free diet

Answer

Pyridoxine and Vitamin B12 supplementation

Answer

Vitamin B1, B6, and B12 supplementation

Question 4

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

Accepted Answer

Serine

Answer

Tyrosine

Answer

Isoleucine

Answer

Phenylalanine

Question 5

Deficiency of arginosuccinate synthase causes which of the following disorders?

Accepted Answer

Citrullinemia

Answer

Hyperargininemia

Answer

Argininosuccinic aciduria

Answer

Type I hyperammonemia

Question 6

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 7

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

Accepted Answer

Homocystinuria

Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Maple syrup disease

Question 8

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 9

What is the most direct precursor of taurine?

Accepted Answer

Cysteine

Answer

Glycine

Answer

Methionine

Answer

Glutathione

Question 10

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

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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