**Core Concept**
Membranous conjunctivitis is a type of conjunctivitis characterized by the formation of a membrane on the conjunctiva, typically due to an infectious or inflammatory process. The causative agent often involves a viral or bacterial pathogen that triggers a strong immune response, leading to the production of a thick, fibrotic membrane.

**Why the Correct Answer is Right**
The correct answer is **Haemophilus influenzae**. This bacterium is a common cause of membranous conjunctivitis, particularly in children. The pathogenesis involves the invasion of the conjunctiva by the bacteria, which triggers an intense inflammatory response. The production of IgG and IgA antibodies against the bacterial antigens leads to the formation of a thick, fibrotic membrane on the conjunctiva. The membrane is composed of fibrin, dead epithelial cells, and inflammatory cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chlamydia trachomatis is a common cause of inclusion conjunctivitis, but it typically presents with follicular conjunctivitis and not membranous conjunctivitis.
* **Option B:** Adenovirus is a common cause of viral conjunctivitis, but it usually presents with a follicular or papillary reaction rather than a membranous membrane.
* **Option D:** Streptococcus pneumoniae can cause conjunctivitis, but it is not the most common cause of membranous conjunctivitis.

**Clinical Pearl / High-Yield Fact**
Haemophilus influenzae type b (Hib) is a common cause of membranous conjunctivitis in children, and its vaccination has significantly reduced the incidence of this condition. The vaccination against Hib is a key preventive measure for membranous conjunctivitis in children.

**Correct Answer:** C. Haemophilus influenzae