A newborn with ophthalmia neonatorum after 3 days is most likely infected with which organism?

Ophthalmia neonatorum is a form of conjunctivitis that occurs in newborns, typically within the first month of life, and can be caused by various pathogens. When evaluating the timing of the presentation, if symptoms appear within the first few days after birth, the most likely causative organism is Neisseria gonorrhoeae.

This bacterium is often transmitted during delivery when the baby passes through an infected birth canal. Symptoms from this organism tend to manifest quickly, often within 2 to 5 days. The classic presentation involves severe purulent discharge and can lead to corneal ulceration if not treated promptly.

Other pathogens, such as Chlamydia trachomatis, while also a potential cause of conjunctivitis in newborns, typically lead to symptoms that arise around the second week of life. Herpes simplex virus infections may present differently and would generally have a more severe clinical picture, often with systemic involvement. Streptococcus pneumoniae is less commonly associated with neonatal conjunctivitis.

Therefore, when considering the organism responsible for conjunctivitis in a newborn occurring just after three days, Neisseria gonorrhoeae is the most probable causative agent due to its timing and clinical presentation.