What condition is characterized by the overproduction of keratin leading to thickened nails?

The condition characterized by the overproduction of keratin, leading to thickened nails, is onychomycosis. This fungal infection of the nails often results in a variety of symptoms, including discoloration, thickening, and potential separation from the nail bed. The overproduction of keratin occurs as the body responds to the fungal invasion, promoting an abnormal thickening of the nail as a defense mechanism.

In contrast, paronychia is an infection of the tissue surrounding the nail, primarily causing redness, swelling, and pain, but not specifically associated with keratin overproduction. Athlete's foot is a fungal infection affecting the feet, particularly the skin, and does not directly involve the nails. Lastly, a fungal nail infection is a more general term that might encompass onychomycosis, but does not specifically denote the thickening of the nails due to keratin overproduction. Thus, the specificity of onychomycosis regarding keratin involvement makes it the most accurate answer.