A 24 year old man had multiple, small hypopigmented macules on the upper chest and back for the last three months. The macules were circular, arranged around follicles and many had coalesced to form large sheets. The surface of the macules showed fine scaling. He had similar lesions one year ago which subsided with treatment. The most appropriate investigation to confirm the diagnosis is -

Multiple small hypopigmented macules on chest & back, fine scales, history of perifollicular arrangement and reappearance after treatment suggest the diagnosis of P. versicolor. KOH 10-20% is used on scaling skin lesions as it dissolves keratin and allows easier visualization of fungal elements. Microscopic examination of the KOH mount reveals sho hyphae and round spores - "spaghetti and meatballs"appearance