Which among the following is the most common cause of neonatal blindness?
Ref. The Short Textbook of Pediatrics. Pg 146
 
Ophthalmia neonatorum (ON)
Purulent conjunctivitis occurring during the first four weeks of life. T

Two most common causative agents are Neisseria gonorrhoeae and Chlamydia trachomatis, the former being of more concern here because of its propensity to cause blindness.
The eyes of the newborn infant are infected during passage through the birth canal of a mother infected with the causative organisms, and symptoms usually appear within the first three days of life.
ON is an ocular emergency and must be diagnosed and treated as quickly as possible.

There are three major strategies for the control of ON:

Prevention of the disease by antenatal treatment of infected parents
Prophylaxis at birth
Treatment of the clinical disease in the newborn.

Prophylaxis - 1% tetracycline eye ointment and 1% silver nitrate eye drops are highly effective in prophylaxis.
Neisseria gonorrhoeae
Chlamydia trachomatis
Klebsiella
Enterobacter