What is the most likely causative agent in a 4-week-old male showing symptoms of conjunctivitis and cough?

In a 4-week-old male presenting with conjunctivitis and cough, the most likely causative agent is Chlamydia. Neonates can contract Chlamydia trachomatis from their mothers during childbirth, leading to conjunctivitis in the first few weeks of life. This bacterial infection can also cause respiratory symptoms, such as a cough, as it may involve the respiratory tract.

Chlamydia's association with conjunctivitis in newborns is well-documented, making it a top consideration in this age group. The timing of the onset of symptoms is typically within the first month post-delivery, aligning with the presentation of this infant.

While other agents like Streptococcus pneumoniae and Mycoplasma pneumoniae could cause respiratory issues and even conjunctivitis, their prevalence and typical age of onset are less associated with neonates. Respiratory syncytial virus (RSV) primarily causes respiratory symptoms in infants but is not commonly responsible for conjunctivitis. Therefore, considering the age and symptoms of the infant, Chlamydia is the most appropriate causative agent in this scenario.