Which of the following is a common disease that can affect the nails?

Fungal infections are indeed a common disease that can affect the nails. These infections typically occur when fungi invade the nail bed, leading to various symptoms such as discoloration, thickening, and brittleness of the nails. Fungal nail infections, also known as onychomycosis, are quite prevalent and can be difficult to treat, often requiring antifungal medications.

While psoriasis, alopecia, and eczema can also have effects on the skin or even the nails, they are less commonly associated specifically with nail pathology as fungal infections are. Psoriasis can cause pitting or changes in nail texture, but it is not classified primarily as a nail disease. Alopecia mainly affects hair and does not directly influence nail health. Eczema is primarily a skin condition that may cause discomfort around the nails but does not commonly result in nail diseases on its own. Thus, fungal infections are recognized as a significant and common issue concerning nail health, making it the correct answer.