**Core Concept**
Megaloblastic anemia is a condition characterized by the production of abnormally large red blood cells that are not able to function properly. This condition is often caused by a deficiency in either vitamin B12 or folate, leading to impaired DNA synthesis and cell division.

**Why the Correct Answer is Right**
The reason megaloblastic anemia should be treated with both folic acid and vitamin B12 is that a deficiency in either of these two nutrients can lead to impaired DNA synthesis and cell division. Vitamin B12 is essential for the synthesis of DNA, while folate is involved in the transfer of single carbon units. If either of these nutrients is deficient, it can lead to an accumulation of homocysteine and methylmalonic acid, which can cause damage to the nervous system and other tissues. Treating megaloblastic anemia with both folic acid and vitamin B12 ensures that the underlying cause of the deficiency is addressed, and that the patient receives adequate nutrition to support DNA synthesis and cell division.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because folic acid alone can mask the symptoms of vitamin B12 deficiency, leading to a delay in diagnosis and treatment of the underlying cause.
**Option B:** This option is incorrect because vitamin B12 alone may not be sufficient to treat megaloblastic anemia caused by folate deficiency.
**Option C:** This option is incorrect because it does not address the underlying cause of megaloblastic anemia, which is a deficiency in either vitamin B12 or folate.

**Clinical Pearl / High-Yield Fact**
It's worth noting that a deficiency in vitamin B12 can lead to a specific type of anemia called pernicious anemia, which is characterized by a lack of intrinsic factor, a protein necessary for vitamin B12 absorption.

**Correct Answer: B. Vitamin B12 is necessary for the synthesis of DNA and is often deficient in patients with megaloblastic anemia, especially those with pernicious anemia.