A 6-week-old male infant presents with fever, irritability,…

A 6-week-old male infant presents with fever, irritability, and poor feeding. Urinalysis is positive for leukocyte esterase and nitrites, and urine culture is pending. The NP considers empiric therapy with trimethoprim–sulfamethoxazole (TMP-SMX). The infant was born at 35 weeks’ gestation and has a history of mild jaundice in the neonatal period. Current labs show total bilirubin 1.2 mg/dL (normal for age), creatinine 0.3 mg/dL, and electrolytes within normal limits. Which concern makes TMP-SMX an inappropriate choice for this patient population?