**Core Concept**
The question tests the knowledge of B Complex vitamins, a group of essential nutrients involved in various bodily functions including energy production, nerve function, and heart health. Deficiency of these vitamins can lead to various diseases.

**Why the Correct Answer is Right**
A deficiency of B Complex vitamins can lead to Beriberi, a disease characterized by weakness, fatigue, and heart problems. The primary cause of Beriberi is a deficiency of Thiamine (Vitamin B1), which plays a crucial role in energy production and nerve function. Thiamine deficiency disrupts the normal functioning of the nervous system, leading to the characteristic symptoms of Beriberi.

**Why Each Wrong Option is Incorrect**
**Option A:** Pellagra, a disease caused by a deficiency of Niacin (Vitamin B3), is characterized by dermatitis, diarrhea, and dementia. While it is a B Complex vitamin deficiency, it is not the correct answer.
**Option B:** A deficiency of Folate (Vitamin B9) can lead to Megaloblastic anemia, a condition characterized by the production of abnormally large red blood cells. This is a different condition from Beriberi.
**Option C:** A deficiency of Vitamin B12 can lead to neurological problems, including numbness, weakness, and cognitive impairment. While it is a B Complex vitamin deficiency, it is not the correct answer.
**Option D:** A deficiency of Riboflavin (Vitamin B2) can lead to Ariboflavinosis, a condition characterized by cracks in the corners of the mouth and inflammation of the tongue. This is a different condition from Beriberi.

**Clinical Pearl / High-Yield Fact**
Beriberi is a classic example of a disease caused by a deficiency of a B Complex vitamin. Thiamine plays a crucial role in energy production, and its deficiency can lead to serious health consequences, including heart problems and neurological damage.

**Correct Answer: A. Beriberi**