A 16-year-old boy comes to the office complaining of itchiness and rash on his upper back for the past week. He says his girlfriend first noticed the rash on his nape seven days ago, which then spread to cover most of his upper back and shoulders. Two weeks ago, he went on a trip with his family to Indonesia and stayed in a beach resort for four days. Past medical history is unremarkable, and he does not take any medications. His pulse is 75/min, respirations are 18/min, blood pressure is 110/75 mm Hg, and his temperature is 37.0°C (98.6°F). Body mass index (BMI) is 25.9 kg/m2. Physical examination shows well-demarcated, hypopigmented macules over the back, chest, and shoulder girdle.
A) Lesion scrapings with potassium hydroxide (KOH)
Pityriasis versicolor, also known as tinea versicolor, is caused by Malassezia globosa, a yeast infection that can inhabit the skin and feed on lipids to survive. It is more common in hot, humid climates because the increased sweating creates a suitable environment where M. globosa can live and thrive. Pityriasis versicolor often presents as pruritic, hypopigmented macules along the areas rich in sebaceous glands like the trunk, neck, and arms. To confirm the diagnosis, lesion scraping with potassium hydroxide (KOH) preparation can be done, and it will reveal hyphae and spores resembling “spaghetti and meatballs”. Treatment may be achieved with topical selenium sulfide.
Answer choice B: Patch testing, is incorrect. Patch testing can be helpful in patients with allergic contact dermatitis. It can aid in identifying offending allergens and allow patients to avoid them. However, this patient’s history makes allergic dermatitis unlikely.
Answer choice C: Skin biopsy, is incorrect. Biopsy is often done for the diagnosis of skin malignancy and vitiligo and in areas of diagnostic uncertainty. The patient’s clinical history and characteristic skin lesions both point towards pityriasis versicolor, so performing a skin biopsy is unwarranted.
Answer choice D: Skin culture, is incorrect. Skin culture can be done to confirm bacterial and fungal skin infections. However, because results take longer to obtain and could delay treatment, more rapid tests such as lesion scrapings with potassium hydroxide are preferred over cultures.
Answer choice E: Wood’s light examination, is incorrect. Wood’s light exam involves shining an ultraviolet (UV) light against the skin; the area infected with M. globosa will emit a yellow-orange fluorescence. However, this finding can only be found in 50% of cases and thus is not an appropriate confirmatory test.
Key Learning Point
Malassezia globosa is the causative agent of Pityriasis versicolor. It is most common in hot, humid areas. It often presents as well-demarcated, pruritic, hypopigmented macules around the torso and arms. Confirmatory diagnosis is done by microscopic examination of skin scrapings with KOH preparation.
