Which of the following conditions is a fungal infection of the nails?

Onychomycosis is indeed a fungal infection of the nails that is primarily caused by various types of fungi, including dermatophytes, yeasts, and non-dermatophyte molds. This condition is characterized by discoloration, thickening, and sometimes crumbling of the nail. It typically affects the toenails more often than the fingernails and can lead to discomfort and further complications if not treated.

In contrast, psoriasis is a chronic skin condition that can cause changes to the nails but is not a fungal infection. It may present with pitting, ridges, and nail separation, but the underlying cause is immune-related rather than infectious.

Athlete's foot is another fungal condition, but it affects the skin between the toes and the feet, not the nails. It's caused by similar fungi that lead to onychomycosis but targets a different area of the body.

A bacterial infection involves bacteria rather than fungi and can manifest in different ways, such as redness, swelling, and discharge, but it does not pertain specifically to nail infections caused by fungal organisms.