Which of the following conditions is NOT a nail disease?

Pterygium is a condition that typically refers to an abnormal growth of tissue that can occur at the nail's cuticle, often leading to the attachment of the nail structure to the underlying skin. While it may affect the nail and its growth, it is not classified as a disease but rather a deformity or condition related to the nail apparatus. In contrast, agnail, onychomycosis, and onycholysis are recognized as nail diseases. Agnail, also known as hangnail, involves the extension of skin around the nail, frequently resulting in pain and inflammation. Onychomycosis is a fungal infection that affects the nail, causing discoloration and often leading to thickening or crumbling. Onycholysis involves the separation of the nail from its nail bed, which can arise from various factors, including injury or fungal infection.