Which of the following nail diseases is considered a contraindication to a pedicure?

Onychomycosis is a fungal infection of the nails that can cause changes in the nail's appearance and integrity, leading to potential discomfort or pain during a pedicure. Since it is contagious and can spread to other nails or to other clients, it poses a significant risk in a salon setting. Clients with onychomycosis are advised against receiving pedicures to prevent spreading the infection and to avoid exacerbating their condition, making it a clear contraindication.

The other conditions, while they may affect the appearance or health of the nails, do not carry the same level of risk for spreading infections or complications during a pedicure. Psoriasis can affect the nails, but it is not contagious and can often be managed with proper care. Hangnails, though painful and unsightly, do not pose a risk to others and can typically be treated during a pedicure with care. Athlete's foot, while also a fungal infection, primarily affects the skin between the toes and does not directly impact the nails in the same way that onychomycosis does.