Dermatophytosis & Superficial Fungal Infections — Topic Test

A 9-month-old infant has well-demarcated, glistening erythematous plaques in the inguinal and axillary folds that fail to respond to topical antifungals and emollients, accompanied by recalcitrant diarrhoea and perioral and acral crusting. Serum alkaline phosphatase is low. Which is the most likely underlying deficiency?