**Core Concept**
Cheilosis, also known as angular stomatitis, is a condition characterized by inflammation and fissuring of the corners of the mouth. It is often associated with nutritional deficiencies, particularly those affecting the skin and mucous membranes.

**Why the Correct Answer is Right**
Cheilosis is commonly linked to deficiencies of essential nutrients such as vitamin B2 (riboflavin), vitamin B3 (niacin), and iron. These nutrients play crucial roles in maintaining healthy skin and mucous membranes. Vitamin B2, in particular, is vital for the synthesis of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), which are essential for energy production in cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Niacin deficiency is indeed associated with cheilosis.
**Option B:** Iron deficiency can lead to cheilosis due to impaired collagen synthesis and poor wound healing.
**Option C:** Riboflavin (vitamin B2) deficiency is a well-known cause of cheilosis, making this option incorrect as a choice for "all except".

**Clinical Pearl / High-Yield Fact**
It's essential to note that cheilosis can also be caused by other factors such as chronic lip licking, iron-deficiency anemia, and certain skin conditions like eczema or psoriasis.

**Correct Answer:**
There seems to be an issue with the question. A correct question would have multiple options and a correct answer.