My cat lost fur on its chin with red spots and crusting—what is it?

Chin fur loss with crusting is likely black chin, also called folliculitis, a common cat disease caused by overactive oil glands blocking pores. First, keep an Elizabethan collar on to prevent scratching, wipe the area with a chlorhexidine solution on a cotton ball, then apply erythromycin ointment 1-2 times daily. Since it often recurs, keep the chin clean after meals to reduce bacteria, use ceramic or stainless bowls, and feed a light diet; for males, neuter at the right age to control male hormones.