A 6-month-old boy presents with a one month history of a rash in his diaper area. Past medical history is notable for recurrent ear infections, for which he has taken several courses of amoxicillin. His parents are concerned that the rash appears to be worsening.

On physical examination, the child appears well and is afebrile, but gets fussy when examining his diaper area.

Which of the following is the most likely diagnosis?