A 6-week-old male infant born to a mother with untreated chlamydial cervicitis presents with a 2-week history of purulent eye discharge and conjunctival injection. The discharge is copious and bilateral. Gram stain of the conjunctival exudate shows no intracellular gram-negative diplococci. The infant has no respiratory symptoms. What is the most likely diagnosis and the recommended first-line antibiotic?