## **Core Concept**
The question describes a patient with hypopigmented scaly macules on the chest and back, and the use of a Wood's lamp for diagnostic purposes. This scenario suggests a fungal infection, specifically **pityriasis versicolor**, which is caused by *Malassezia* species. A Wood's lamp is a tool used in dermatology to help diagnose fungal and bacterial infections.

## **Why the Correct Answer is Right**
The correct answer, **C. Yellow**, is associated with the fluorescence seen under a Wood's lamp for *Malassezia* infections, which cause **pityriasis versicolor**. When exposed to ultraviolet (UV) light from a Wood's lamp, the lesions of pityriasis versicolor typically exhibit a yellowish fluorescence due to the production of **porphyrins** by the *Malassezia* yeast.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify a color; thus, it cannot be evaluated as a correct response.
- **Option B:** This option is incorrect because it does not specify a color; thus, it cannot be evaluated as a correct response.
- **Option D:** This option suggests a color but is not the correct answer. Without specifying the color, we know that for conditions like **vitiligo**, the Wood's lamp typically shows a blue or purple fluorescence due to the absence of melanin, not yellow.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **pityriasis versicolor**, caused by *Malassezia* species, is characterized by **yellow fluorescence** under a Wood's lamp. This condition commonly presents with hypopigmented or hyperpigmented scaly macules on the chest and back, particularly in young adults. Remember, *Malassezia* species are part of the normal skin flora but can cause disease under certain conditions.

## **Correct Answer:** . Yellow