**Core Concept**
Niacin deficiency, also known as pellagra, is a disease caused by a lack of the vitamin niacin (vitamin B3) in the diet. In maize-eating populations, this deficiency often occurs due to the lack of a specific amino acid in the diet.

**Why the Correct Answer is Right**
In maize (corn), the amino acid tryptophan is present in a bound form, making it unavailable for the body to use. Tryptophan is a precursor to niacin, and the body can convert it into niacin through a series of enzyme-catalyzed reactions. However, this conversion cannot occur with the bound form of tryptophan found in maize. As a result, maize-eating populations are at risk of developing pellagra if they do not consume sufficient amounts of tryptophan or niacin from other sources.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the specific issue of tryptophan deficiency in maize-eating populations.
**Option B:** This option is incorrect because it is not related to the cause of niacin deficiency in maize-eating populations.
**Option C:** This option is incorrect because it is a symptom of niacin deficiency, not the cause.

**Clinical Pearl / High-Yield Fact**
In maize-eating populations, the addition of tryptophan to the diet can prevent pellagra by providing the necessary amino acid for niacin synthesis.

**Correct Answer: A. Tryptophan deficiency in maize.**