Which of the following is a common symptom of a fungal nail infection?

Discoloration and thickening of the nail is a hallmark indicator of a fungal nail infection. When a fungal infection occurs, it disrupts the normal physiology of the nail, often leading to changes in color; nails may appear yellow, brown, or even green. Additionally, the growth patterns of the nail can be affected, resulting in thickening as the infected tissue becomes abnormally keratinized. This thickening occurs because the fungus stimulates excessive keratin production as the nail attempts to defend itself against the infection.

The other symptoms listed do not typically indicate a fungal infection. For example, increased shine is more often associated with healthy nails or the use of certain products, and excessive flexibility is not a characteristic of fungal infections, which usually cause nails to become brittle or hard. Moreover, rapid growth of the nail is generally unrelated to fungal infections, as such infections usually slow down nail growth or can cause nails to become distorted in shape.