Pityriasis Versicolor Hypopigmented

A 22-year-old male college student presents to the dermatology clinic in late summer with multiple white spots on his upper back and chest that became noticeable after returning from a spring break beach vacation. The spots are asymptomatic but aesthetically concerning, as they became more obvious when the surrounding skin tanned but the spots did not. Physical examination reveals the findings marked as **A** in the diagram — multiple well-demarcated hypopigmented macules and patches in a seborrheic distribution. The patient recalls similar patches in prior summers that resolved over winter months and admits to wearing sweaty workout clothes for extended periods. Which of the following best explains the pathophysiology of the hypopigmentation seen in the lesions marked **A**?