## Core Concept
The question pertains to identifying a disease caused by a deficiency of a vitamin B-complex. Vitamin B-complex includes several B vitamins that play crucial roles in various bodily functions, including energy production, nerve function, and heart health. Deficiencies in these vitamins can lead to specific diseases.

## Why the Correct Answer is Right
The correct answer, , is associated with **Pellagra**, a disease caused by a lack of **Niacin (Vitamin B3)**, which is part of the B-complex vitamins. Pellagra is characterized by the "three Ds": diarrhea, dermatitis, and dementia. Niacin is crucial for the synthesis of NAD+ and NADP+, which are essential for redox reactions and maintaining the health of the skin, nervous system, and gastrointestinal tract.

## Why Each Wrong Option is Incorrect
- **Option A:** is related to **Scurvy**, which is caused by a deficiency of **Vitamin C**, not a B-complex vitamin. Scurvy leads to symptoms like fatigue, swollen gums, joint pain, and poor wound healing.
- **Option B:** corresponds to **Rickets**, a disease caused by a deficiency of **Vitamin D**, leading to softening and weakening of bones in children. Vitamin D is crucial for calcium absorption and bone health.
- **Option C:** is associated with **Beriberi**, which is caused by a deficiency of **Thiamine (Vitamin B1)**, another B-complex vitamin. Beriberi affects the heart, digestive system, and the nervous system.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **Pellagra** is specifically linked to **Niacin deficiency** and is classically described by the "three Ds." This association is critical for diagnosing and managing patients with suspected nutritional deficiencies.

## Correct Answer Line
**Correct Answer: D. Pellagra**