A 24-year old male had multiple, small hypopigmented macules on the upper chest and hack for 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
KOH preparation Slit skin smear Tzanck test Skin biopsy