Clinical nutrition and hematology focus Vitamin B12 (cobalamin) is useful in the prevention and treatment of which specific deficiency-related disorder in humans? Provide the best single answer.

Introduction / Context:
Vitamin B12 (cobalamin) is a water-soluble vitamin critical for DNA synthesis, methylation reactions, and neurologic function. A classic exam point is distinguishing which diseases respond to B12 versus those that require other vitamins. This question targets recognition of the correct clinical indication for B12 in prevention and treatment.

Given Data / Assumptions:

• Pernicious anemia is caused by autoimmune destruction of gastric parietal cells or intrinsic factor, leading to B12 malabsorption.
• Scurvy is due to vitamin C deficiency; beri-beri is due to thiamin (vitamin B1) deficiency.
• Cataract is not treated by vitamin B12 supplementation.

Concept / Approach:
The core concept is vitamin–disease matching. B12 deficiency leads to megaloblastic anemia with neurologic features. In pernicious anemia, parenteral B12 bypasses the intrinsic factor–dependent absorption step, correcting hematologic abnormalities and preventing neurologic complications.

Step-by-Step Solution:
Identify the deficiency disease linked to B12: megaloblastic anemia with neurologic dysfunction.Determine the cause: pernicious anemia involves impaired intrinsic factor and poor B12 absorption.Treatment strategy: administer B12 (typically intramuscular or high-dose oral) to prevent/treat anemia and neuropathy.Exclude other options that correspond to different vitamins or unrelated conditions.

Verification / Alternative check:
Laboratory findings such as elevated mean corpuscular volume, low serum B12, elevated methylmalonic acid and homocysteine, plus anti-intrinsic factor antibodies, support the diagnosis and the response to B12 therapy.

Why Other Options Are Wrong:
Scurvy: treated with vitamin C, not B12.Cataract: multifactorial; B12 is not a standard therapy.Beri-beri: due to thiamin deficiency; treat with vitamin B1.Megaloblastic anemia due to folate deficiency only: requires folate; B12 alone will not correct isolated folate deficiency and may mask neurologic issues if misused.

Common Pitfalls:
Confusing folate with B12; assuming any megaloblastic anemia equals folate deficiency; overlooking neurologic signs unique to B12 deficiency.

Final Answer:
Pernicious anemia.