A 40-year-old chronic alcoholic presented to the OPD with complaints of B/L distal lower limb numbness and paresthesias along with weakness and dizziness. He has a history of stroke in the past. On examination, Characteristic skin lesions were observed Impaired vibration and proprioception Preserved pain and temperature Sensory ataxia Lab findings revealed a low EAST-AC value. Which of the following reactions uses the above deficient vitamin as coenzyme:-

**Core Concept**
Vitamin B12 plays a crucial role in the synthesis of myelin, the maintenance of the nervous system, and the metabolism of fatty acids and amino acids. A deficiency in vitamin B12 can lead to neurological symptoms such as numbness, weakness, and ataxia, as seen in this patient.

**Why the Correct Answer is Right**
The patient's presentation and lab findings suggest a deficiency in vitamin B12, which is essential for the activity of the enzyme methionine synthase. This enzyme is involved in the conversion of homocysteine to methionine, a process that requires vitamin B12 as a coenzyme. The patient's low EAST-AC (Erythrocyte Acetylcholinesterase) value is a marker of vitamin B12 deficiency, as this enzyme is dependent on the availability of vitamin B12 for its activity. The characteristic skin lesions observed in this patient may be indicative of subacute combined degeneration, a condition associated with vitamin B12 deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the reaction involving folate as a coenzyme is the conversion of homocysteine to methionine, which is catalyzed by the enzyme methionine synthase, but folate is not the deficient vitamin in this scenario.

**Option B:** This option is incorrect because the reaction involving pyridoxine (Vitamin B6) as a coenzyme is the conversion of amino acids, but this does not directly relate to the patient's symptoms or lab findings.

**Option C:** This option is incorrect because the reaction involving thiamine (Vitamin B1) as a coenzyme is the decarboxylation of alpha-keto acids in the Krebs cycle, but this does not directly relate to the patient's symptoms or lab findings.

**Option D:** This option is incorrect because the reaction involving riboflavin (Vitamin B2) as a coenzyme is the transfer of electrons in the electron transport chain, but this does not directly relate to the patient's symptoms or lab findings.

**Clinical Pearl / High-Yield Fact**
Vitamin B12 deficiency can lead to neurological symptoms such as numbness, weakness, and ataxia, and can be diagnosed using lab tests such as the EAST-AC value. This condition is often associated with gastrointestinal malabsorption, particularly in patients with a history of chronic alcoholism.

**Correct Answer: C.**

✓ Correct Answer: D. All of the above