A 60-year-old patient taking long-term proton pump inhibitors develops fatigue and paresthesias. You suspect B12 deficiency due to impaired absorption. Which laboratory pattern would best support this diagnosis?

• A. Low serum B12, elevated methylmalonic acid, and elevated homocysteine — consistent with B12 deficiency from reduced gastric acid/intrinsic factor function.
• B. Elevated urinary biotin and undetectable serum vitamin C, which are specific indicators of B12 deficiency due to drug interactions.
• C. Low serum niacin with markedly elevated LDL cholesterol and decreased HDL, which is the hallmark lab pattern of B12 malabsorption.
• D. High serum folate with low methylmalonic acid and normal homocysteine, indicating pure folate toxicity rather than B12 deficiency.

Answer: (A)