A 6-month-old infant is brought to the clinic for a worsening diaper rash for 5 days. The parents have been applying zinc oxide barrier cream with minimal improvement. The infant recently completed a 7-day course of amoxicillin for acute otitis media. Physical examination shows beefy, erythematous, confluent plaques in the diaper area with multiple small erythematous papules and pustules at the periphery. The rash involves the inguinal skin folds. The infant is otherwise well appearing and afebrile.

Which of the following is the most likely diagnosis?