Light micrograph of fungal hyphae (threads) in a sample taken from a patient with ringworm (tinea). Ringworm is a fungal infection of the skin that is highly contagious. This sample was taken from a patient with tinea incognita, a form of the infection that has been incorrectly treated with steroids rather than an antifungal agent. The sample has undergone a KOH prep test, which is used to diagnose a fungal infection. Potassium hydroxide (KOH) is added to the material on the slide to dissolve any skin cells. The sample is then examined under a light microscope for the presence of fungal cells.