Question

A patient with vitamin B12 deficiency presents with anemia, positive Romberg sign, and elevated homocysteine. Which amino acid is most likely deficient?

• A. Cysteine
• B. Methionine
• C. Tyrosine
• D. Glutamate

Hint:

Vitamin B12 deficiency causes increased homocysteine because homocysteine cannot be converted efficiently to methionine.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
Vitamin B12 is a cofactor for the remethylation of homocysteine.
The question asks which amino acid becomes deficient when this specific B12-dependent reaction is blocked.
Step 2: Detailed Explanation:

The B12-dependent pathway: In the cytoplasm, the enzyme Methionine Synthase uses Vitamin B12 (as methylcobalamin) to transfer a methyl group from 5-methyl-THF to Homocysteine.

Products of the reaction: This reaction produces Methionine and regenerates Tetrahydrofolate (THF).

Effect of B12 Deficiency: When B12 is absent, this reaction cannot proceed.

Resulting Biochemical Profile:

The substrate, Homocysteine, accumulates in the blood (hyperhomocysteinemia).

The product, Methionine, cannot be synthesized, leading to its deficiency.


Clinical Correlation: Methionine is a precursor for S-adenosylmethionine (SAM), which is required for the methylation of myelin basic protein. A lack of SAM leads to demyelination of the spinal cord (Subacute Combined Degeneration), explaining the positive Romberg sign.

Analyzing Options: Among the choices, Methionine is the direct product of the B12-dependent remethylation cycle.

Step 3: Final Answer:
Vitamin B12 deficiency prevents the conversion of homocysteine into methionine, thereby leading to methionine deficiency and elevated homocysteine levels.