Amino Acid Metabolism Practice Questions

Q: Which of the following is a key gluconeogenic amino acid?

Alanine. **Explanation:** **Alanine** is the correct answer because it is the primary and most important gluconeogenic amino acid in the body. This is due to the **Cahill Cycle (Glucose-Alanine Cycle)**. During fasting or exercise, muscle protein is broken down; the resulting amino groups are transferred to pyruvate to form Alanine. Alanine is then transported to the liver, where it is deaminated back into pyruvate, serving as a direct substrate for gluconeogenesis. **Analysis of Options:** * **Serine (Option B):** While Serine is a glucogenic amino acid (converted to pyruvate), it is not considered the "key" or primary transporter of carbon skeletons from muscle to liver like Alanine. * **Valine (Option C):** Valine is a purely glucogenic branched-chain amino acid (BCAA) that enters the TCA cycle via Succinyl-CoA. Though important, it is not the predominant gluconeogenic precursor. * **Tyrosine (Option D):** Tyrosine is **both glucogenic and ketogenic**. It is a precursor for catecholamines and thyroid hormones but is not a primary substrate for glucose synthesis. **High-Yield NEET-PG Pearls:** * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). They cannot be converted to glucose. * **Both Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, Isoleucine (Mnemonic: **PITTT**). * **Key Enzyme:** Alanine Aminotransferase (ALT/SGPT) requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor for the interconversion of Alanine and Pyruvate. * **Clinical Fact:** In states of starvation, Alanine is the first amino acid to be depleted from the muscle to maintain blood glucose levels.

Q: Which of the following amino acids is not ketogenic?

Methionine. **Explanation:** Amino acids are classified based on their metabolic end-products into three categories: **Glucogenic** (converted to glucose), **Ketogenic** (converted to acetyl-CoA or acetoacetate), or **Both**. **1. Why Methionine is the Correct Answer:** Methionine is a **purely glucogenic** amino acid. Its metabolism leads to the formation of **Succinyl-CoA**, a key intermediate of the TCA cycle, which can then be used for gluconeogenesis. It does not produce ketone bodies or acetyl-CoA. **2. Analysis of Incorrect Options:** * **Leucine (A) & Lysine (B):** These are the only two **purely ketogenic** amino acids. They are metabolized directly into acetyl-CoA or acetoacetate and cannot be used to synthesize glucose. * **Tryptophan (D):** This is **both glucogenic and ketogenic**. Its breakdown yields pyruvate (glucogenic) and acetoacetyl-CoA (ketogenic). Other amino acids in this dual category include Isoleucine, Phenylalanine, and Tyrosine (Mnemonic: **PITTT** – Phenylalanine, Isoleucine, Tyrosine, Tryptophan, Threonine). **High-Yield Clinical Pearls for NEET-PG:** * **Purely Ketogenic:** Leucine and Lysine (The "L" amino acids). * **Both Glucogenic & Ketogenic:** Phenylalanine, Isoleucine, Tyrosine, Tryptophan, Threonine. * **Purely Glucogenic:** All remaining 13 amino acids (including Methionine, Valine, and Histidine). * **Clinical Correlation:** In cases of **Maple Syrup Urine Disease (MSUD)**, the metabolism of branched-chain amino acids (Leucine, Isoleucine, Valine) is impaired. Among these, Leucine is the most potent ketogenic amino acid and contributes significantly to neurological symptoms.

Q: Abnormal mousy/mushy odour of urine is associated with which condition?

Phenylketonuria. **Explanation:** The correct answer is **Phenylketonuria (PKU)**. PKU is an autosomal recessive inborn error of metabolism caused by a deficiency of the enzyme **phenylalanine hydroxylase**, which converts phenylalanine to tyrosine. This leads to the accumulation of phenylalanine and its metabolites, such as **phenylacetic acid**, in the urine. The characteristic **mousy or musty odor** is specifically attributed to phenylacetic acid. **Analysis of Options:** * **Phenylketonuria (Correct):** Associated with a mousy/musty odor, intellectual disability, seizures, and hypopigmentation (fair skin/blue eyes) due to decreased melanin synthesis. * **Tyrosinemia:** Characterized by a **boiled cabbage-like** or **rancid butter** odor. It often presents with liver failure, renal tubular dysfunction (Fanconi syndrome), and rickets. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the branched-chain alpha-keto acid dehydrogenase complex. It results in a distinct **burnt sugar or maple syrup** odor of the urine. * **Hawkinsuria:** A rare defect in tyrosine metabolism (4-hydroxyphenylpyruvate dioxygenase deficiency) that typically presents with a **swimming pool or chlorine-like** odor. **Clinical Pearls for NEET-PG:** * **Isovaleric Acidemia:** Sweaty feet odor. * **Trimethylaminuria:** Fishy odor. * **Multiple Carboxylase Deficiency:** Tomcat urine odor. * **Oasthouse Urine Disease (Methionine Malabsorption):** Dried malt or hops odor. * **Screening:** The **Guthrie Test** (bacterial inhibition assay) is the classic screening method for PKU, though Tandem Mass Spectrometry (TMS) is now the gold standard.

Q: Inherited hyperammonemia is a result of deficiency of which enzyme of the Krebs-Henseleit urea cycle?

N-acetyl glutamate synthetase. ### Explanation **Correct Option: C. N-acetyl glutamate synthetase (NAGS)** The **Krebs-Henseleit cycle (Urea Cycle)** is the primary mechanism for detoxifying ammonia into urea. The first and rate-limiting enzyme of this cycle is **Carbamoyl Phosphate Synthetase I (CPS-I)**. However, CPS-I is inactive without its essential allosteric activator, **N-acetylglutamate (NAG)**. NAG is synthesized from acetyl-CoA and glutamate by the enzyme **N-acetylglutamate synthetase (NAGS)**. A deficiency in NAGS leads to a failure to activate CPS-I, resulting in a complete block of the urea cycle. This causes a toxic buildup of ammonia in the blood (**Hyperammonemia**), leading to encephalopathy and cerebral edema. **Analysis of Incorrect Options:** * **A, B, and D (Malate, Isocitrate, and Succinate Dehydrogenase):** These are enzymes of the **TCA Cycle (Krebs Cycle)**, not the Urea Cycle. While the TCA cycle provides the ATP and aspartate required for the urea cycle, deficiencies in these specific enzymes do not typically present with isolated primary hyperammonemia. **Clinical Pearls for NEET-PG:** * **Most Common Urea Cycle Disorder:** Ornithine Transcarbamoylase (OTC) deficiency (X-linked recessive). * **NAGS Deficiency Mimicry:** NAGS deficiency clinically mimics CPS-I deficiency. Both present with hyperammonemia and **low/absent orotic acid** levels. * **Treatment:** NAGS deficiency is unique because it can be treated with **Carglumic acid** (an analog of N-acetylglutamate), which directly activates CPS-I. * **Mnemonic:** "NAGS is the key that unlocks CPS-I."

Q: A 2-month-old infant presents with poor feeding, vomiting, and lethargy. Laboratory studies show hyperammonemia and elevated levels of orotic acid in the urine. Which of the following is the most likely enzyme deficiency?

Ornithine transcarbamylase (OTC) deficiency. ***Ornithine transcarbamylase (OTC) deficiency*** - **Most common X-linked urea cycle disorder** presenting in infancy with hyperammonemia - **Classic triad:** Hyperammonemia + elevated urinary orotic acid + low blood urea nitrogen - **Biochemical mechanism:** OTC converts carbamoyl phosphate + ornithine → citrulline. When deficient, carbamoyl phosphate accumulates and spills into the pyrimidine synthesis pathway, producing excess orotic acid - Clinical features: poor feeding, vomiting, lethargy, seizures, developmental delay *Carbamoyl phosphate synthetase I (CPS I) deficiency* - Also causes hyperammonemia but **NO orotic aciduria** (defect occurs before OTC step) - Carbamoyl phosphate is not formed, so cannot enter pyrimidine pathway - Cannot be distinguished clinically from OTC deficiency without measuring orotic acid *Argininosuccinate lyase deficiency* - Causes hyperammonemia with elevated **argininosuccinate** in blood/urine - Does not cause orotic aciduria (defect is downstream of OTC) - May present with trichorrhexis nodosa (brittle hair) *Orotidine 5′-phosphate decarboxylase deficiency* - Causes hereditary **orotic aciduria** but **NO hyperammonemia** (not a urea cycle disorder) - Defect in de novo pyrimidine synthesis pathway - Presents with megaloblastic anemia, growth retardation, responds to uridine supplementation

Q: A patient presents with arthritis, and arthroscopy reveals blackening of the joints. Which enzyme deficiency is most likely responsible for this condition?

Homogentisate oxidase. ***Homogentisate oxidase***- Deficiency of this enzyme causes **Alkaptonuria**, resulting in the accumulation of **homogentisic acid**, which oxidizes into dark polymeric pigment called **ochronotic pigment**.- This pigment deposits in cartilage and connective tissues (**Ochronosis**), leading to severe destructive arthritis, especially in large joints, and causing the characteristic **blackening of joint tissues** observed on arthroscopy.*Branched-chain alpha-ketoacid dehydrogenase*- Deficiency of this enzyme causes **Maple Syrup Urine Disease (MSUD)**, leading to the accumulation of **branched-chain amino acids** (leucine, isoleucine, valine).- MSUD is characterized by severe neurologic dysfunction and a distinctive maple syrup odor in the urine, not chronic arthritis or joint pigmentation.*Tyrosinase*- This enzyme is crucial for the synthesis of **melanin** from tyrosine.- Deficiency results in conditions like **Albinism**, characterized by hypopigmentation of the skin, hair, and eyes, which is distinct from ochronotic arthritis.*Fumarase*- Deficiency of this enzyme (Fumarate hydratase) impairs the **Krebs cycle** (TCA cycle).- Clinical features include severe intellectual disability, microcephaly, and metabolic acidosis, without the development of ochronosis or black joints.

Question 1

Which of the following is a key gluconeogenic amino acid?

Accepted Answer

Alanine

Answer

Serine

Answer

Valine

Answer

Tyrosine

Question 2

A 6-month-old infant presents with lethargy, vomiting, poor feeding, convulsions, and severe psychomotor retardation. Physical examination reveals microcephaly with prominent maxillae, widely spaced teeth, blue irises, seborrheic or eczematoid rash, spasticity, hyperreflexia, and tremor. A musty odor is noted in the urine. What is the most likely diagnosis, and which of the following statements about the disease is FALSE?

Accepted Answer

The complications can be prevented by restricting the dietary intake of the involved amino acid.

Answer

The disease may be caused by a deficiency of the cofactor tetrahydrobiopterin (BH4).

Answer

Tandem mass spectrometry is useful in the diagnosis.

Answer

Pegvaliase can be administered to adults suffering from this condition.

Question 3

Which of the following amino acids is not ketogenic?

Accepted Answer

Methionine

Answer

Leucine

Answer

Lysine

Answer

Tryptophan

Question 4

Which of the following clinical laboratory observations is suggestive of Maple syrup urine disease?

Accepted Answer

High fecal levels of tryptophan and indole derivatives

Answer

Burnt sugar smell in urine

Answer

High plasma phenylalanine levels

Answer

Extremely high levels of citrulline in urine

Question 5

Diagnosis of carcinoid tumors is done by?

Accepted Answer

5-HIAA

Answer

DHEA

Answer

DHEA

Answer

Metanephrines

Question 6

A 1-year-old child presents with delayed milestones. Examination reveals fair skin with hypopigmented hair and blue eyes. The child also has eczema and is irritable. What is the most likely diagnosis?

Accepted Answer

Phenylketonuria

Answer

Albinism

Answer

Alkaptonuria

Answer

Cystinosis

Question 7

Abnormal mousy/mushy odour of urine is associated with which condition?

Accepted Answer

Phenylketonuria

Answer

Tyrosinemia

Answer

Maple syrup urine disease

Answer

Hawkinsuria

Question 8

Inherited hyperammonemia is a result of deficiency of which enzyme of the Krebs-Henseleit urea cycle?

Accepted Answer

N-acetyl glutamate synthetase

Answer

Malate dehydrogenase

Answer

Isocitrate dehydrogenase

Answer

Succinate dehydrogenase

Question 9

A 2-month-old infant presents with poor feeding, vomiting, and lethargy. Laboratory studies show hyperammonemia and elevated levels of orotic acid in the urine. Which of the following is the most likely enzyme deficiency?

Accepted Answer

Ornithine transcarbamylase (OTC) deficiency

Answer

Orotidine 5′-phosphate decarboxylase deficiency

Answer

Carbamoyl phosphate synthetase I deficiency

Answer

Argininosuccinate lyase deficiency

Question 10

A patient presents with arthritis, and arthroscopy reveals blackening of the joints. Which enzyme deficiency is most likely responsible for this condition?

Accepted Answer

Homogentisate oxidase

Answer

Fumarase

Answer

Tyrosinase

Answer

Branched-chain alpha-ketoacid dehydrogenase

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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