Potassium Hydroxide (KOH) preparation is a rapid microscopic test used for the preliminary detection of yeast and fungal elements in clinical samples. The KOH solution (usually 10-20% concentration) dissolves and digests human epithelial cells and keratinous material (like skin, hair, nails) in the sample, while leaving fungal structures (hyphae or yeast cells) intact and clear for microscopic examination.