**Core Concept**
Niacin deficiency, also known as pellagra, is a disease caused by a lack of vitamin B3 (nicotinic acid) in the diet. It is characterized by the "three Ds": diarrhea, dermatitis, and dementia.

**Why the Correct Answer is Right**
Pellagra occurs due to the body's inability to synthesize niacin from tryptophan, an amino acid found in protein-rich foods. The enzyme kynureninase is crucial in this process, but its activity is impaired in individuals with niacin deficiency. This leads to a buildup of toxic metabolites, which cause the characteristic symptoms of pellagra. The pathophysiology involves the disruption of tryptophan metabolism, resulting in the accumulation of kynurenine and its derivatives, which are toxic to the skin, nervous system, and gastrointestinal tract.

**Why Each Wrong Option is Incorrect**
**Option A:** Pellagra is indeed characterized by diarrhea, making this option incorrect.
**Option B:** Dermatitis is a hallmark symptom of pellagra, so this option is also incorrect.
**Option C:** Dementia is another key feature of pellagra, making this option incorrect as well.

**Clinical Pearl / High-Yield Fact**
Pellagra is often associated with a diet that is deficient in tryptophan, which can occur in individuals with a diet that is high in maize (corn) and low in protein. This is because maize lacks the amino acid tryptophan, leading to a niacin deficiency.

**Correct Answer: D.**