In the treatment of undiagnosed megaloblastic anemia, vitamin B12 and folic acid should be given together because:

**Core Concept**
Megaloblastic anemia is a condition caused by impaired DNA synthesis in red blood cell precursors, leading to their abnormal growth and accumulation in the bone marrow. This is often due to deficiencies in either vitamin B12 or folic acid, which are crucial for DNA synthesis.

**Why the Correct Answer is Right**
Vitamin B12 and folic acid deficiencies both result in impaired DNA synthesis. However, their mechanisms differ: vitamin B12 is necessary for the methylation of deoxyuridine monophosphate (dUMP) to thymidine monophosphate (dTMP), a process mediated by the enzyme methionine synthase. Folic acid, on the other hand, is required for the conversion of dUMP to dTMP, a reaction catalyzed by the enzyme dihydrofolate reductase (DHFR). When both vitamins are deficient, their combined effects on DNA synthesis can lead to severe megaloblastic anemia. Administering both vitamins together is crucial to restore normal DNA synthesis and prevent further bone marrow suppression.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incomplete, but even if it were a specific statement, it would likely be incorrect without context.
**Option B:** There is no direct evidence to suggest that giving one vitamin and withholding the other would improve outcomes in megaloblastic anemia.
**Option C:** This statement is misleading, as vitamin B12 and folic acid deficiencies have distinct pathophysiological mechanisms, making it incorrect to imply a direct equivalence.

**Clinical Pearl / High-Yield Fact**
In patients with megaloblastic anemia, it is essential to determine the underlying cause (vitamin B12 or folic acid deficiency) before initiating treatment to avoid masking the diagnosis and potentially leading to irreversible neurological damage in vitamin B12 deficiency.

**Correct Answer: C. Because both deficiencies can lead to impaired DNA synthesis, and giving both together helps restore normal hematopoiesis.**

✓ Correct Answer: B. Folic acid alone causes improvement of anemic symptoms but neurological dysfunction continues.