**Core Concept**
Ophthalmia neonatorum refers to conjunctivitis or keratoconjunctivitis that occurs in the first month of life. The condition is often caused by infections, and it requires prompt diagnosis and treatment to prevent vision loss and other complications.

**Why the Correct Answer is Right**
The most common cause of ophthalmia neonatorum is Neisseria gonorrhoeae, a gram-negative diplococcus. This bacterium is typically acquired by the newborn during passage through an infected birth canal. If left untreated, N. gonorrhoeae can cause severe conjunctivitis and potentially lead to corneal perforation and vision loss. The pathogenesis involves the bacterium's ability to colonize the conjunctiva and cause an inflammatory response.

**Why Each Wrong Option is Incorrect**
**Option A:** Chlamydia trachomatis is a common cause of ophthalmia neonatorum, but it is not the most common cause. Chlamydial conjunctivitis typically presents with a slower onset and less severe symptoms compared to gonococcal conjunctivitis.
**Option B:** Herpes simplex virus (HSV) is a rare cause of ophthalmia neonatorum. While HSV keratitis can be severe, it is not the most common cause of this condition.
**Option C:** Allergic conjunctivitis is not typically a cause of ophthalmia neonatorum, as it is usually an allergic reaction to environmental allergens or substances.

**Clinical Pearl / High-Yield Fact**
Neisseria gonorrhoeae conjunctivitis in the newborn is a medical emergency that requires prompt treatment with antibiotics, such as ceftriaxone.

**Correct Answer: D. Neisseria gonorrhoeae**