Homocystinuria and Methionine Metabolism — MCQs

10 questions

A young boy with a thin build and long fingers presents with diminished vision. On examination, subluxation of the lens is observed, and cystathionine synthase deficiency is detected. Which amino acid should the patient be supplemented with?

A patient with homocystinuria presents with ectopia lentis (dislocation of the lens). Which vitamin should be supplemented?

In the context of homocystinuria, which direction does the lens typically subluxate?

What is the form of folate that accumulates in vitamin B12 deficiency due to the inhibition of methionine synthase?

A 45-year-old patient presents with joint pain and weakness and is known to have homocystinuria. Which vitamin is required in the treatment?

Methionine can enter the TCA cycle at which level?

Which one among the following essential amino acids is usually the "limiting" amino acid in most of the pulses?

Which of the following is a lipotropic factor?

Which of the following amino acids is not converted to succinyl-CoA?

Q10

Which amino acid requires ascorbic acid for its formation in the body?

Homocystinuria and Methionine Metabolism Indian Medical PG Practice Questions and MCQs

Question 1: A young boy with a thin build and long fingers presents with diminished vision. On examination, subluxation of the lens is observed, and cystathionine synthase deficiency is detected. Which amino acid should the patient be supplemented with?

A. Tyrosine
B. Serine
C. Cysteine (Correct Answer)
D. Methionine
E. Glycine

Explanation: ***Cysteine*** - In **cystathionine synthase deficiency** (homocystinuria), the body cannot convert **homocysteine to cystathionine**, and subsequently to **cysteine**. - **Cysteine** therefore becomes an **essential amino acid** in these patients and must be supplemented. *Tyrosine* - **Tyrosine** is a non-essential amino acid synthesized from **phenylalanine**, and its deficiency is not directly related to cystathionine synthase deficiency. - It is not involved in the **methionine or homocysteine metabolic pathway** that is disrupted in homocystinuria. *Serine* - **Serine** is a substrate for the **cystathionine synthase enzyme**, which combines with **homocysteine** to form **cystathionine**. - Supplementation with serine alone would not bypass the enzyme deficiency or provide the essential product, **cysteine**. *Methionine* - **Methionine** is the precursor to **homocysteine**, and in cystathionine synthase deficiency, there's often an accumulation of methionine and homocysteine. - Therefore, **methionine restriction** is typically part of the treatment, not supplementation. *Glycine* - **Glycine** is involved in one-carbon metabolism but is not directly involved in the **transsulfuration pathway** affected by cystathionine synthase deficiency. - Glycine supplementation would not address the inability to synthesize **cysteine** from homocysteine.

Question 2: A patient with homocystinuria presents with ectopia lentis (dislocation of the lens). Which vitamin should be supplemented?

A. Vitamin B12
B. Vitamin B6 (Correct Answer)
C. Thiamine
D. Vitamin B9 (Folate)
E. Vitamin C

Explanation: ***Vitamin B6*** - Many cases of **homocystinuria** are due to a deficiency in **cystathionine beta-synthase (CBS)**, an enzyme that requires **pyridoxal phosphate (active B6)** as a cofactor. - Supplementing with **high doses of vitamin B6** can significantly improve outcomes in patients with **B6-responsive homocystinuria** by increasing residual CBS enzyme activity. *Vitamin B12* - While vitamin B12 (cobalamin) is a cofactor for **methionine synthase**, an enzyme involved in homocysteine metabolism, it is not the primary treatment for **homocystinuria due to CBS deficiency**. - B12 deficiency can lead to increased homocysteine levels, but B12 supplementation alone will not address the underlying enzyme defect in most cases of homocystinuria. *Thiamine* - **Thiamine (Vitamin B1)** is essential for carbohydrate metabolism and nerve function, but it has no direct role in the metabolic pathway involved in homocystinuria. - Thiamine deficiency causes **beriberi** and **Wernicke-Korsakoff syndrome**, symptoms distinctly different from homocystinuria. *Vitamin B9 (Folate)* - **Folate** is a cofactor for **methionine synthase**, working alongside vitamin B12 to convert homocysteine back to methionine. - While folate deficiency can contribute to hyperhomocysteinemia, it is not the primary therapeutic intervention for homocystinuria caused by **CBS deficiency**. *Vitamin C* - **Vitamin C (ascorbic acid)** is important for collagen synthesis and acts as an antioxidant, but it has no role in homocysteine metabolism or the treatment of homocystinuria. - Vitamin C deficiency causes **scurvy**, which presents with bleeding gums, petechiae, and poor wound healing—completely unrelated to homocystinuria.

Question 3: In the context of homocystinuria, which direction does the lens typically subluxate?

A. Inferotemporal
B. Inferonasal (Correct Answer)
C. Superonasal
D. Superotemporal

Explanation: ***Inferonasal*** - In **homocystinuria**, the **ectopia lentis** (lens subluxation) often occurs due to weakening of the **zonular fibers**. - The classic direction for lens subluxation in homocystinuria is **inferior and nasal**. *Inferotemporal* - While lens subluxation can occur in various directions, **inferotemporal** is not the classic or most common presentation in homocystinuria. - This direction is less specific and does not strongly point to homocystinuria as the underlying cause. *Superonasal* - **Superonasal** dislocation of the lens is more characteristic of **Marfan syndrome**, which is important to differentiate from homocystinuria. - This direction helps distinguish different causes of lens ectopia. *Superotemporal* - **Superotemporal** lens subluxation is the hallmark of **Marfan syndrome**, a genetic connective tissue disorder. - This specific finding is crucial for differential diagnosis in patients presenting with lens ectopia.

Question 4: What is the form of folate that accumulates in vitamin B12 deficiency due to the inhibition of methionine synthase?

A. Methyl-THFA (Correct Answer)
B. Formyl THFA
C. Methylene-THFA
D. Methenyl THFA

Explanation: ***Methyl-THFA*** - In **vitamin B12 deficiency**, the enzyme **methionine synthase** is inhibited, as it requires B12 as a cofactor. - This leads to the accumulation of **methyl-THFA (5-methyltetrahydrofolate)** because it cannot be converted back to tetrahydrofolate (THF), trapping it in this form—known as the "**methyl-trap hypothesis**." *Formyl THFA* - **Formyl THFA (10-formyltetrahydrofolate)** is a folate derivative involved in purine synthesis, not directly accumulated due to B12 deficiency. - Its levels are not specifically noted to increase in the context of methionine synthase inhibition. *Methylene-THFA* - **Methylene-THFA (5,10-methylenetetrahydrofolate)** is an important substrate for **thymidylate synthase** in DNA synthesis. - While its metabolism is linked to folate, it does not accumulate as the primary "methyl-trap" form in B12 deficiency. *Methenyl THFA* - **Methenyl THFA (5,10-methenyltetrahydrofolate)** is an intermediate in folate metabolism, interconverting with methylene-THFA and formyl-THFA. - Like other folate derivatives not directly involved in the inhibited methionine synthase step, it doesn't accumulate specifically in B12 deficiency.

Question 5: A 45-year-old patient presents with joint pain and weakness and is known to have homocystinuria. Which vitamin is required in the treatment?

A. Vitamin B6 (Correct Answer)
B. Vitamin B12
C. Vitamin B7
D. Vitamin B1
E. Vitamin B9

Explanation: ***Vitamin B6*** - **Homocystinuria** is often caused by a deficiency in the enzyme **cystathionine beta-synthase**, which requires **pyridoxal phosphate (active form of B6)** as a cofactor. - Supplementation with high-dose **vitamin B6** can help some patients by increasing the residual activity of the enzyme, thereby reducing **homocysteine levels**. - This is the **primary treatment** for **B6-responsive homocystinuria** (approximately 50% of cases respond to B6 therapy). *Vitamin B12* - Vitamin B12 is a cofactor for the enzyme **methionine synthase**, which converts homocysteine back to methionine. - While it plays a role in homocysteine metabolism, **vitamin B6** is typically the primary treatment for homocystinuria caused by **cystathionine beta-synthase deficiency**. *Vitamin B9* - Vitamin B9 (folic acid) works together with **vitamin B12** as a cofactor in the **remethylation pathway** via methionine synthase. - While folate supplementation may help lower homocysteine levels, it is **not the primary treatment** for classical homocystinuria due to cystathionine beta-synthase deficiency. - **Vitamin B6** remains the first-line vitamin therapy for enzyme deficiency-related homocystinuria. *Vitamin B7* - Vitamin B7, or **biotin**, is a cofactor for carboxylase enzymes and is involved in fatty acid synthesis and gluconeogenesis. - It has no direct role in the metabolism of **homocysteine** or the treatment of homocystinuria. *Vitamin B1* - Vitamin B1, or **thiamine**, is essential for carbohydrate metabolism and nerve function. - It is not involved in the metabolic pathways that regulate **homocysteine levels** or the treatment of homocystinuria.

Question 6: Methionine can enter the TCA cycle at which level?

A. Fumarate
B. Oxaloacetate
C. Succinyl-CoA (Correct Answer)
D. Citrate

Explanation: ***Succinyl - CoA*** - Methionine is a **glucogenic amino acid** that is catabolized to propionyl-CoA, which is then converted to **methylmalonyl-CoA** and finally to **succinyl-CoA**. - **Succinyl-CoA** is an intermediate of the **TCA cycle**, allowing methionine-derived carbons to enter the cycle. *Fumarate* - Fumarate is an intermediate of the TCA cycle, but methionine catabolism does not directly produce **fumarate**. - Amino acids like **phenylalanine** and **tyrosine** can be catabolized to fumarate. *Oxaloacetate* - **Oxaloacetate** is a TCA cycle intermediate and can be formed from **pyruvate** (via pyruvate carboxylase) or from certain amino acids like **aspartate** and **asparagine**. - Methionine does not directly convert to oxaloacetate. *Citrate* - **Citrate** is the first intermediate formed in the TCA cycle when **acetyl-CoA** combines with **oxaloacetate**. - Methionine catabolism does not lead to the direct formation of citrate.

Question 7: Which one among the following essential amino acids is usually the "limiting" amino acid in most of the pulses?

A. Valine
B. Threonine
C. Methionine (Correct Answer)
D. Lysine

Explanation: ***Methionine*** - In **pulses** (legumes), the sulfur-containing amino acids, primarily **methionine** and **cysteine**, are often the first limiting amino acids. - This means that the amount of protein synthesis that can occur is restricted by the availability of methionine, even if other essential amino acids are abundant. *Valine* - **Valine** is an essential branched-chain amino acid, but it is typically not the limiting amino acid in pulses. - It is more likely to be limiting in certain grains or in specific diets lacking a variety of protein sources. *Threonine* - **Threonine** is an essential amino acid that can be limiting in some cereals, but it is not typically the primary limiting amino acid in pulses. - Its deficiency is less common in a balanced diet including legumes. *Lysine* - **Lysine** is often the limiting amino acid in most **cereal grains** (e.g., wheat, corn, rice). - Pulses, however, are generally good sources of lysine, making its deficiency less likely when consumed.

Question 8: Which of the following is a lipotropic factor?

A. Sphingomyelin
B. Histidine
C. Bilirubin
D. Methionine (Correct Answer)

Explanation: ***Methionine*** - **Methionine** is an essential amino acid that serves as a precursor for **choline** and **creatine**, both of which play crucial roles in lipid metabolism and transport. - Lipotropic factors prevent or reverse the accumulation of **fat in the liver** by promoting the synthesis of **lipoproteins**, which package and transport fats from the liver to other tissues. *Sphingomyelin* - **Sphingomyelin** is a type of **sphingolipid**, a component of cell membranes and myelin sheaths, but it does not directly act as a lipotropic factor to prevent fatty liver. - While it's involved in cellular signaling and membrane structure, it does not directly facilitate the metabolism or transport of **hepatic triglycerides** in the same way as lipotropic agents. *Histidine* - **Histidine** is an essential amino acid involved in protein synthesis and the production of **histamine**, but it is not considered a primary lipotropic factor. - Its main roles are in **immune response** and **neurotransmission**, not in preventing fat accumulation in the liver. *Bilirubin* - **Bilirubin** is a waste product from the breakdown of **heme**, primarily from red blood cells. It is excreted by the liver. - It is known for its **antioxidant properties** but does not play a direct role as a lipotropic factor in lipid metabolism or in preventing **fatty liver**.

Question 9: Which of the following amino acids is not converted to succinyl-CoA?

A. Methionine
B. Histidine (Correct Answer)
C. Valine
D. Isoleucine

Explanation: ***Histidine*** - Histidine is exclusively converted to **α-ketoglutarate**, a different **TCA cycle intermediate**, via **formiminoglutamate (FIGLU)**. - Its catabolic pathway does not involve formation of **succinyl-CoA**. *Methionine* - Methionine is a glucogenic amino acid that is catabolized to **succinyl-CoA** through several intermediate steps. - These steps include the formation of **S-adenosylmethionine (SAM)** and subsequent conversion to **homocysteine**. *Isoleucine* - Isoleucine is both a glucogenic and ketogenic amino acid. - Its catabolism yields **acetyl-CoA** and **propionyl-CoA**, with the latter being converted to **succinyl-CoA**. *Valine* - Valine is a **branched-chain amino acid** that is exclusively **glucogenic**. - Its breakdown pathway produces **propionyl-CoA**, which is then further metabolized to **succinyl-CoA**.

Question 10: Which amino acid requires ascorbic acid for its formation in the body?

A. Lysine
B. Hydroxyproline (Correct Answer)
C. Cysteine
D. Proline

Explanation: ***Hydroxyproline*** - **Ascorbic acid (Vitamin C)** is an essential cofactor for **prolyl hydroxylase** and **lysyl hydroxylase** enzymes - These enzymes catalyze the **post-translational hydroxylation** of proline and lysine residues within collagen chains to form hydroxyproline and hydroxylysine - This hydroxylation is crucial for **stabilization of the collagen triple helix** structure - Hydroxyproline is formed by **modification of proline after incorporation into collagen**, not as a free amino acid - **Scurvy** (Vitamin C deficiency) results in defective collagen due to inadequate hydroxyproline formation *Lysine* - Lysine is an **essential amino acid** obtained from diet - Does not require ascorbic acid for its synthesis or formation - While lysine residues in collagen can be hydroxylated (forming hydroxylysine), the question asks about the amino acid whose formation requires Vitamin C *Cysteine* - Cysteine is a **sulfur-containing amino acid** synthesized from methionine via transsulfuration pathway - Its synthesis does not involve ascorbic acid *Proline* - Proline is a **non-essential amino acid** synthesized from glutamate - **Proline synthesis does not require ascorbic acid** - Proline serves as the precursor that gets hydroxylated to hydroxyproline within collagen

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free