**Core Concept**
Pityriasis versicolor, a common skin condition caused by Malassezia yeast, is characterized by erythematous lesions that exhibit a distinctive coral red fluorescence under the Wood's lamp due to the accumulation of porphyrins produced by the yeast.

**Why the Correct Answer is Right**
The correct answer is associated with the production of porphyrins, which are a result of the yeast's metabolism. Malassezia yeast feeds on the sebum produced by the sebaceous glands in the skin, leading to the production of these porphyrins. The Wood's lamp emits ultraviolet (UV) light, which causes the porphyrins to fluoresce, resulting in a characteristic coral red color.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the Wood's lamp typically used to diagnose fungal infections like pityriasis versicolor does not produce the specific type of light needed to diagnose this condition.
**Option B:** This is incorrect because the lesions associated with this condition are typically not painful or pruritic, unlike those seen in other skin conditions.
**Option C:** This is incorrect because the Wood's lamp is not typically used to diagnose conditions like eczema or psoriasis, which have different diagnostic criteria.

**Clinical Pearl / High-Yield Fact**
The presence of coral red fluorescence under the Wood's lamp is a classic diagnostic feature of pityriasis versicolor, and it is essential to consider this condition in patients presenting with erythematous lesions in the axilla.

**Correct Answer:** C. Pityriasis versicolor