I have a single discolored toenail. What could this be?

There are many causes of a discolored toenail or thickened nails. A frequent cause of a discolored toenail, thickening, and pain is fungus (onychomycosis). Nail fungus may initially begin beneath the edge of the nail, spread to the entire nail plate other toenails and to the skin as athlete’s foot. Oral antifungals are currently the treatment of choice for fungal nail infections. Other causes of painful, thickened, or discolored toenails could include nail dystrophy, psoriasis, glomus tumors, subungual hematomas and even melanoma.