Answer:
Nick is probably suffering from a fungal infection.
Explanation:
Nail ringworm or onychomycosis, as it is also known scientifically, is an infection caused by fungi such as mold or yeast, which makes the nail yellow, deformed and thick. That is, this is a situation of fungal infection whose main symptoms are deformation, thickening and color change of the nail.
Nick notices thickening and discoloration of the index finger nail, probably Nick is suffering from a fungal infection.
