Final answer:
The correct answer is ceftriaxone, which is a cephalosporin antibiotic frequently used as the first choice to treat gonorrhea in infants, particularly when acquired from an untreated mother.
Step-by-step explanation:
The treatment for an infant born to an untreated mother with gonorrhea is ceftriaxone. Cephalosporins like ceftriaxone are often the first choice for treating gonorrhea due to antibiotic resistance to other antibiotics such as fluoroquinolones. Moreover, the Centers for Disease Control and Prevention (CDC) recommends treating for co-infection with Chlamydia trachomatis using a combination of ceftriaxone and azithromycin. Although erythromycin eye drops were once commonly used to prevent neonatal conjunctivitis resulting from bacteria acquired during vaginal delivery, the focus in the question is on systemic treatment, where ceftriaxone is the preferred choice for systemic infections such as gonorrhea in infants.