**Core Concept**
Ophthalmia neonatorum refers to conjunctivitis or keratoconjunctivitis occurring within the first month of life. It can be caused by various pathogens, including bacteria, viruses, and chlamydia. The timing of presentation is crucial in determining the likely cause.

**Why the Correct Answer is Right**
Infection occurring within 24 hours of birth is most commonly due to maternal gonococcal or chlamydial conjunctivitis. These pathogens are transmitted through the birth canal during delivery. Chlamydia trachomatis is a common cause of inclusion conjunctivitis in newborns, characterized by the presence of characteristic inclusion bodies in conjunctival scrapings. Gonococcal conjunctivitis presents as a severe form of conjunctivitis with purulent discharge and is often accompanied by systemic symptoms.

**Why Each Wrong Option is Incorrect**
* **Option A:** Neisseria meningitidis is a rare cause of ophthalmia neonatorum and typically presents later than 24 hours after birth.
* **Option B:** Herpes simplex virus (HSV) can cause ophthalmia neonatorum, but it typically presents later than 24 hours after birth, often with systemic symptoms and a vesicular rash.
* **Option D:** Pseudomonas aeruginosa is a common cause of conjunctivitis in contact lens wearers but is not typically associated with ophthalmia neonatorum in the neonatal period.

**Clinical Pearl / High-Yield Fact**
The diagnosis of ophthalmia neonatorum often requires a combination of clinical examination, Gram stain, and culture. It is essential to perform a thorough maternal history, including questions about sexually transmitted infections, to guide further investigation.

**Correct Answer: C. Chlamydia trachomatis**