**Core Concept**
Niacin, also known as vitamin B3, is an essential nutrient for maintaining healthy skin, nerves, and the digestive system. A deficiency in niacin can lead to various clinical manifestations, primarily due to its role in the synthesis of NAD and NADP, which are critical for energy metabolism and maintaining the integrity of the skin and mucous membranes.

**Why the Correct Answer is Right**
Dactylitis, or sausage digit, is a condition characterized by inflammation of an entire finger or toe. It is typically associated with conditions like psoriatic arthritis, reactive arthritis, or infections, rather than niacin deficiency. Niacin deficiency primarily affects the skin, causing dermatitis and potentially other mucocutaneous manifestations like diarrhea. However, dactylitis is not a recognized symptom of niacin deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** Dermatitis is a known symptom of niacin deficiency, as seen in pellagra, a condition caused by a lack of niacin in the diet.
**Option C:** Diarrhea can also occur in niacin deficiency due to its role in maintaining the integrity of the mucous membranes in the gastrointestinal tract.
**Option D:** Dementia is a less common manifestation of niacin deficiency, particularly in individuals with chronic alcoholism or malnutrition.

**Clinical Pearl / High-Yield Fact**
In the context of pellagra, the four Ds are a useful mnemonic to remember the classic symptoms of niacin deficiency: dermatitis, diarrhea, dementia, and death, which can occur if left untreated.

**✓ Correct Answer: B. Dactylitis**