Which condition is characterized by thick and brittle nails with white powdery deposits?

The condition characterized by thick and brittle nails with white powdery deposits is indeed onychomycosis. This is a fungal infection that affects the nails, leading to various symptoms, including discoloration, thickening, and changes in texture. The presence of white powdery deposits is particularly indicative of a fungal infection, where the nail plate becomes compromised due to the growth of fungi beneath it.

Onychomycosis can affect both toenails and fingernails, and it results in nails that appear dull and may have an irregular surface. The thickening occurs as the body attempts to fight off the infection and repair the nail structure, resulting in brittle nails that may easily break or peel.

In contrast, the other options relate to different skin or hair conditions. For example, tinea corporis is a fungal infection of the skin, commonly known as ringworm, and does not directly affect the nails. Alopecia areata is an autoimmune condition that results in hair loss and does not involve nails at all. Tinea unguium, while it sounds similar, is actually a synonym for onychomycosis; however, in common parlance, it often refers specifically to the nail infection caused by dermatophytes, whereas onychomycos