**Core Concept**
Ophthalmia neonatorum is a conjunctivitis that occurs in newborns within the first 30 days of life, often resulting from an infection or chemical exposure during delivery.

**Why the Correct Answer is Right**
The most common cause of ophthalmia neonatorum is **Neisseria gonorrhoeae**, a Gram-negative bacterium that is sexually transmitted from mother to child during delivery. This bacterium produces an enzyme called **beta-lactamase**, which confers resistance to many antibiotics. The infection can cause severe conjunctivitis, potentially leading to vision loss if left untreated. **Neisseria gonorrhoeae** is more likely to cause ophthalmia neonatorum than other pathogens due to its high virulence and ability to colonize the eye.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Chlamydia trachomatis** is another common cause of ophthalmia neonatorum, but it is not the most common cause.
* **Option B:** **Herpes simplex virus** can also cause ophthalmia neonatorum, but it is less common than Neisseria gonorrhoeae.
* **Option D:** **Staphylococcus aureus** is a common cause of bacterial conjunctivitis in adults, but it is not the most common cause of ophthalmia neonatorum in newborns.

**Clinical Pearl / High-Yield Fact**
It is essential to include ophthalmia neonatorum in the differential diagnosis of conjunctivitis in newborns, especially if the mother has a history of sexually transmitted infections. Early treatment with antibiotics is crucial to prevent vision loss.

**Correct Answer: C. Neisseria gonorrhoeae**