**Core Concept**
Beri-Beri is a disease caused by a deficiency of a specific vitamin in the diet, leading to various neurological, cardiovascular, and musculoskeletal symptoms. This disease is prevalent in regions with limited access to a balanced diet and is often associated with poverty and malnutrition.

**Why the Correct Answer is Right**
Thiamine, also known as Vitamin B1, plays a crucial role in carbohydrate metabolism and the production of ATP in the body. A deficiency in thiamine leads to impaired glucose metabolism, resulting in the accumulation of toxic intermediates that damage the nervous system and heart. Beri-Beri is characterized by symptoms such as weakness, weight loss, and edema, which are ultimately caused by the depletion of thiamine stores in the body.

**Why Each Wrong Option is Incorrect**
**Option B:** Folic acid is another essential B vitamin that plays a critical role in DNA synthesis and repair. However, its deficiency is associated with megaloblastic anemia rather than Beri-Beri.
**Option C:** Riboflavin, or Vitamin B2, is involved in energy production and can cause ariboflavinosis if deficient. However, this condition is characterized by symptoms such as cracks in the corners of the mouth, dermatitis, and anemia, which are distinct from those of Beri-Beri.
**Option D:** Niacin, or Vitamin B3, is essential for energy metabolism and can cause pellagra if deficient. Pellagra is characterized by the "3 Ds" – diarrhea, dermatitis, and dementia – which differ from the symptoms of Beri-Beri.

**Clinical Pearl / High-Yield Fact**
In areas where thiamine deficiency is common, administering a single dose of thiamine can help alleviate symptoms of Beri-Beri, particularly in cases of acute deficiency. This is because thiamine stores are rapidly depleted, and supplementation can provide a quick fix.

**✓ Correct Answer: A. Thiamine**