Metabolism of Individual Amino Acids — MCQs
Which amino acids accumulate in maple syrup urine disease?
Because free ammonia in the blood is toxic to the body, it is transported in which of the following non-toxic forms?
An infant presented with vomiting, malnutrition, blue eyes, blonde hair & fair skin. On investigation, Guthrie test was positive. All are true regarding this disease EXCEPT:
Phenylketonuria is due to a deficiency of:
In a patient with maple syrup urine disease, all of the following amino acids should be restricted in diet except?
Which of the following is not a substrate for gluconeogenesis?
Which of the following compounds is not formed with the involvement of glycine?
Which dietary deficiency of a vitamin can cause pellagra?
Pyridoxine deficiency leads to altered metabolism of?
A 2-month-old infant presents with vomiting, lethargy, and metabolic acidosis. Blood tests reveal elevated levels of ammonia. What is the most likely diagnosis?
Metabolism of Individual Amino Acids Indian Medical PG Practice Questions and MCQs
Question 1: Which amino acids accumulate in maple syrup urine disease?
- A. Valine
- B. Leucine
- C. Isoleucine
- D. All branched-chain amino acids (Correct Answer)
Explanation: ***All branched-chain amino acids*** - Maple syrup urine disease (MSUD) is characterized by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex**, which is responsible for the breakdown of branched-chain amino acids (BCAAs). - This deficiency leads to the accumulation of **leucine, isoleucine, and valine**, along with their corresponding alpha-keto acids, in the blood and urine. - The distinctive **maple syrup odor** in the urine is caused by the accumulation of branched-chain keto acids derived from all three BCAAs. *Leucine* - While leucine is one of the BCAAs that accumulates in MSUD, it is not the *only* amino acid involved. - The accumulation of **leucine** is particularly associated with the severe neurological symptoms seen in MSUD, as it is the most neurotoxic of the three BCAAs. *Valine* - Valine is another BCAA that accumulates due to the metabolic block in MSUD. - However, the disease involves the accumulation of all three BCAAs, not just valine in isolation. *Isoleucine* - Isoleucine is the third BCAA that accumulates in MSUD due to the defective enzyme. - Like leucine and valine, isoleucine and its corresponding keto acid accumulate in blood and urine when the branched-chain alpha-keto acid dehydrogenase complex is deficient.
Question 2: Because free ammonia in the blood is toxic to the body, it is transported in which of the following non-toxic forms?
- A. Glutamine and urea
- B. Alanine and glutamine (Correct Answer)
- C. Phenylalanine and methionine
- D. Histidine and urea
Explanation: ***Alanine and glutamine*** - **Glutamine** is the primary transporter of ammonia from most peripheral tissues to the liver and kidneys, where it can be safely released and processed. - **Alanine** transports ammonia from muscles to the liver via the glucose-alanine cycle, allowing for hepatic urea synthesis and glucose production. *Glutamine and urea* - While **glutamine** is a major ammonia transporter, **urea** is the *end product* of ammonia detoxification and is primarily synthesized in the liver for excretion. - Urea itself is not a transport form of ammonia *within* the blood before its synthesis; rather, it is the waste product. *Phenylalanine and methionine* - **Phenylalanine** and **methionine** are essential amino acids involved in protein synthesis and various metabolic pathways. - They do not serve as significant carriers for the transport of toxic free ammonia in the bloodstream. *Histidine and urea* - **Histidine** is an amino acid involved in protein synthesis and histamine production, but not primarily in ammonia transport. - As mentioned, **urea** is the final excretory form of detoxified ammonia, not a transport form within the blood.
Question 3: An infant presented with vomiting, malnutrition, blue eyes, blonde hair & fair skin. On investigation, Guthrie test was positive. All are true regarding this disease EXCEPT:
- A. Phenyl acetate positive in urine
- B. Mental retardation is present
- C. Hypopigmentation due to tryptophan deficiency (Correct Answer)
- D. Due to PAH enzyme defect
Explanation: ***Hypopigmentation due to tryptophan deficiency*** - The characteristic **hypopigmentation** (fair skin, blonde hair, blue eyes) in **phenylketonuria (PKU)** is due to **tyrosine deficiency**, not tryptophan deficiency. - **Phenylalanine hydroxylase (PAH)** deficiency leads to accumulation of phenylalanine, which cannot be converted to **tyrosine**. - **Tyrosine** is the precursor for **melanin synthesis** via the enzyme **tyrosinase**, so tyrosine deficiency results in decreased melanin production and hypopigmentation. *Phenyl acetate positive in urine* - In **phenylketonuria (PKU)**, **phenylalanine** accumulates and is shunted to alternative metabolic pathways, leading to the production and excretion of **phenylacetate, phenylpyruvate, and phenyllactate** in the urine. - The presence of these metabolites gives the urine a characteristic **mousey or musty odor**. *Mental retardation is present* - If **phenylketonuria (PKU)** is left untreated, the accumulation of **phenylalanine** is neurotoxic and leads to severe, **irreversible intellectual disability** and **developmental delay**. - Early detection through newborn screening (the **Guthrie test** detects elevated blood phenylalanine) and dietary phenylalanine restriction are crucial to prevent this outcome. *Due to PAH enzyme defect* - **Phenylketonuria (PKU)** is primarily caused by a deficiency in the enzyme **phenylalanine hydroxylase (PAH)**, which is responsible for converting phenylalanine to tyrosine. - This **autosomal recessive genetic disorder** leads to the accumulation of phenylalanine in the blood and tissues, causing the clinical manifestations.
Question 4: Phenylketonuria is due to a deficiency of:
- A. Phenylalanine hydroxylase (PAH) (Correct Answer)
- B. Galactokinase
- C. Tyrosinase
- D. Phenylalanine
Explanation: ***Phenylalanine hydroxylase (PAH)*** - **Phenylketonuria (PKU)** is an autosomal recessive disorder caused by a deficiency of the enzyme **phenylalanine hydroxylase (PAH)**. - This enzyme is crucial for converting the amino acid **phenylalanine** to **tyrosine**. *Phenylalanine* - Phenylalanine is the **substrate** that accumulates in PKU due to the enzyme deficiency, not the deficiency itself. - High levels of phenylalanine are **toxic** to the brain and lead to the clinical manifestations of PKU. *Galactokinase* - Deficiency of **galactokinase** is associated with **galactosemia type II**, a disorder of galactose metabolism. - This condition is characterized by **cataracts** and typically does not involve the neurologic symptoms seen in PKU. *Tyrosinase* - **Tyrosinase** deficiency is the primary cause of **oculocutaneous albinism type 1**, affecting melanin synthesis. - It results in hypopigmentation of the skin, hair, and eyes, which is unrelated to PKU.
Question 5: In a patient with maple syrup urine disease, all of the following amino acids should be restricted in diet except?
- A. Methionine (Correct Answer)
- B. Isoleucine
- C. Leucine
- D. Valine
Explanation: ***Methionine*** - **Maple syrup urine disease (MSUD)** is a disorder affecting the metabolism of **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine. - Therefore, methionine, which is not a BCAA, typically does not need to be restricted and is, in fact, an **essential amino acid** crucial for protein synthesis. *Isoleucine* - **Isoleucine** is a branched-chain amino acid (BCAA) whose metabolism is impaired in MSUD due to a deficiency in **branched-chain alpha-keto acid dehydrogenase complex**. - Accumulation of isoleucine and its corresponding alpha-keto acid is toxic and must be **restricted in the diet**. *Leucine* - **Leucine** is another branched-chain amino acid (BCAA) that cannot be properly metabolized in MSUD. - High levels of leucine and its metabolites are particularly **neurotoxic** and contribute to the characteristic neurological symptoms, necessitating strict dietary restriction. *Valine* - **Valine** is the third branched-chain amino acid (BCAA) whose breakdown is defective in MSUD. - Dietary restriction of valine is essential to prevent its accumulation, which can lead to metabolic crises and **developmental delays**.
Question 6: Which of the following is not a substrate for gluconeogenesis?
- A. Leucine (Correct Answer)
- B. Lactate
- C. Propionate
- D. Glycerol
Explanation: ***Leucine*** - **Leucine** is an exclusively **ketogenic amino acid**, meaning its breakdown products can only be converted into **ketone bodies** or fatty acids, not glucose. - It does not have a carbon skeleton that can be directly converted into **pyruvate** or **oxaloacetate**, which are key intermediates in gluconeogenesis. *Lactate* - **Lactate** is a major substrate for gluconeogenesis, particularly during exercise or fasting. - It is converted to **pyruvate** by **lactate dehydrogenase**, and pyruvate can then enter the gluconeogenic pathway. *Propionate* - **Propionate** is a fatty acid with an odd number of carbon atoms, primarily derived from the catabolism of odd-chain fatty acids or from bacterial fermentation in the colon. - It can be converted into **succinyl CoA**, an intermediate of the citric acid cycle, which can then be used for gluconeogenesis. *Glycerol* - **Glycerol**, released during the breakdown of triglycerides, is an important substrate for gluconeogenesis. - It is phosphorylated to **glycerol-3-phosphate**, which is then oxidized to **dihydroxyacetone phosphate (DHAP)**, an intermediate in glycolysis and gluconeogenesis.
Question 7: Which of the following compounds is not formed with the involvement of glycine?
- A. Purines
- B. Glutathione
- C. Thyroxine (Correct Answer)
- D. Heme
Explanation: ***Thyroxine*** - **Thyroxine** (and other thyroid hormones) are derived from the amino acid **tyrosine**. - Their synthesis involves iodination and coupling reactions of tyrosine residues within the protein **thyroglobulin**. *Heme* - **Glycine** is a direct precursor for the initial step in **heme synthesis**. - It condenses with **succinyl CoA** to form α-amino-β-ketoadipate, which then decarboxylates to form δ-aminolevulinate (ALA). *Purines* - **Glycine** contributes part of its structure to the **purine ring**. - Specifically, the **nitrogen at position 7** and the **carbons at positions 4 and 5** of the purine ring are derived from glycine. *Glutathione* - **Glutathione** is a tripeptide composed of three amino acids: **glutamate**, **cysteine**, and **glycine**. - **Glycine** is the C-terminal amino acid of glutathione and is essential for its structure and function as an antioxidant.
Question 8: Which dietary deficiency of a vitamin can cause pellagra?
- A. Niacin (Correct Answer)
- B. Vitamin A
- C. Folate
- D. Vitamin B12
Explanation: ***Niacin*** - **Pellagra** is classically described by the "4 D's": **dermatitis**, **diarrhea**, **dementia**, and eventually **death**, all of which are caused by a deficiency in **niacin (Vitamin B3)**. - Niacin is crucial for energy metabolism and DNA repair, and its deficiency affects rapidly dividing cells, especially in the skin, gastrointestinal tract, and nervous system. *Vitamin A* - Deficiency in **Vitamin A** primarily leads to **vision problems**, including **night blindness** and, in severe cases, **xerophthalmia** and irreversible blindness. - While it can impact immune function and epithelial integrity, it does not cause the characteristic triad of pellagra. *Folate* - A deficiency in **folate (Vitamin B9)** is primarily associated with **megaloblastic anemia**, which involves enlarged, immature red blood cells. - It does not cause the dermatological, gastrointestinal, or neurological symptoms seen in pellagra. *Vitamin B12* - **Vitamin B12 (cobalamin) deficiency** also causes **megaloblastic anemia**, but it is uniquely associated with **neurological symptoms** such as peripheral neuropathy, cognitive impairment, and subacute combined degeneration of the spinal cord. - While there is some neurological overlap, its clinical presentation is distinct from pellagra and does not involve the characteristic dermatitis or severe diarrhea.
Question 9: Pyridoxine deficiency leads to altered metabolism of?
- A. Phenylalanine
- B. Methionine
- C. Tyrosine
- D. Tryptophan (Correct Answer)
Explanation: ***Tryptophan*** - **Pyridoxine (vitamin B6)** is a critical coenzyme in the metabolism of **tryptophan**, particularly in its conversion to **niacin** and serotonin. - A deficiency leads to an accumulation of abnormal tryptophan metabolites, such as **xanthurenic acid**, which can be excreted in the urine. *Phenylalanine* - The metabolism of phenylalanine involves its conversion to tyrosine, a process catalyzed by **phenylalanine hydroxylase**, which does not directly require pyridoxine. - Deficiencies in phenylalanine metabolism often point to issues like **phenylketonuria**. *Methionine* - Methionine metabolism involves a cycle that generates **S-adenosylmethionine (SAM)** and then homocysteine. - While vitamin B6 is involved in the transsulfuration pathway (converting homocysteine to cysteine), its primary direct impact on methionine metabolism itself is less pronounced than on tryptophan. *Tyrosine* - Tyrosine is synthesized from phenylalanine and is a precursor for **catecholamines** and thyroid hormones. - Its metabolism does not directly rely on pyridoxine as a coenzyme in the main initial steps.
Question 10: A 2-month-old infant presents with vomiting, lethargy, and metabolic acidosis. Blood tests reveal elevated levels of ammonia. What is the most likely diagnosis?
- A. Urea Cycle Disorder (Correct Answer)
- B. Phenylketonuria
- C. Galactosemia
- D. Maple Syrup Urine Disease
Explanation: ***Urea Cycle Disorder*** - The combination of **vomiting**, **lethargy**, metabolic acidosis, and **elevated ammonia** in a 2-month-old infant is highly indicative of a urea cycle disorder due to the impaired detoxification of ammonia. - **Ammonia is neurotoxic**, explaining the lethargy, and its accumulation leads to severe metabolic derangements. *Phenylketonuria* - Characterized by the inability to metabolize **phenylalanine**, leading to its accumulation and neurological damage. - While it can cause developmental delay and seizures, it typically does **not present with acute metabolic acidosis** or hyperammonemia in infancy. *Galactosemia* - An inherited disorder of **galactose metabolism** that can cause vomiting, lethargy, and liver dysfunction, but is typically associated with **jaundice**, hepatomegaly, and **reducing substances in urine**, not primarily hyperammonemia and metabolic acidosis as the presenting features. - The primary defect is in the conversion of galactose to glucose, not ammonia detoxification. *Maple Syrup Urine Disease* - A rare metabolic disorder caused by a defect in the metabolism of **branched-chain amino acids** (leucine, isoleucine, valine). - Presents with feeding difficulties, lethargy, and a characteristic **sweet-smelling urine** ("maple syrup" odor), which is not mentioned in this case.
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