**Core Concept**
Vitamin B12 deficiency leads to a complex set of symptoms due to its crucial role in DNA synthesis, fatty acid metabolism, and the maintenance of myelin sheaths in the nervous system. It is essential for the proper functioning of the nervous system and the formation of red blood cells.

**Why the Correct Answer is Right**
The correct answer is Vitamin B12 (Cyanocobalamin). The deficiency of Vitamin B12 causes neurological damage, including neuritis, due to demyelination of nerve fibers in the peripheral nervous system. This is because Vitamin B12 is necessary for the synthesis of myelin, the fatty substance that surrounds and protects nerve fibers. Furthermore, Vitamin B12 plays a crucial role in the synthesis of DNA, which is essential for cell division and growth. A deficiency in Vitamin B12 can lead to megaloblastic anemia, characterized by the production of large, immature red blood cells. Additionally, Vitamin B12 deficiency can cause skin hyperpigmentation, particularly in the creases of the skin, due to the accumulation of melanin.

**Why Each Wrong Option is Incorrect**
* **Option A:** Vitamin A deficiency causes night blindness, dry skin, and impaired immune function, but it is not associated with the symptoms described in the question.
* **Option B:** Vitamin D deficiency can cause rickets in children and osteomalacia in adults, but it does not lead to the specific symptoms mentioned, such as perioral rash and neuritis.
* **Option D:** Folate deficiency can also cause megaloblastic anemia, but it is not associated with the same range of neurological symptoms as Vitamin B12 deficiency.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Vitamin B12 deficiency can be caused by a lack of dietary intake, impaired absorption (e.g., due to gastric surgery or celiac disease), or increased demand (e.g., during pregnancy or lactation). A deficiency in Vitamin B12 can be treated with oral or intramuscular supplements.

**Correct Answer:** C. Cyanocobalamin.