**Question:** Pseudomembranous conjunctivitis is caused by

A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Haemophilus influenzae
D. Clostridium difficile

**Core Concept:** Pseudomembranous conjunctivitis is a form of bacterial conjunctivitis characterized by the presence of a yellowish-white, fibrous membrane on the conjunctival surface of the eye. It is usually a complication of antibiotic therapy or gastrointestinal infection.

**Why the Correct Answer is Right:** Pseudomembranous conjunctivitis is caused by Clostridium difficile, an anaerobic gram-positive bacillus that is part of the normal gut flora. It is a significant pathogen when its spores are activated by the disruption of the normal gut flora, usually due to prior antibiotic usage. In this case, C. difficile produces toxins (A and B) that lead to inflammation and the formation of the characteristic fibrous membrane on the conjunctiva.

**Why Each Wrong Option is Incorrect:**

A. Staphylococcus aureus: This is a common cause of bacterial conjunctivitis but is not specifically associated with the formation of the fibrous membrane seen in pseudomembranous conjunctivitis.

B. Streptococcus pneumoniae: While S. pneumoniae can cause bacterial conjunctivitis, it is not the primary causative agent for pseudomembranous conjunctivitis.

C. Haemophilus influenzae: Similar to S. pneumoniae, H. influenzae can cause bacterial conjunctivitis but is not directly associated with pseudomembranous conjunctivitis.

**Clinical Pearl:** Pseudomembranous conjunctivitis is a rare but serious complication of antibiotic therapy. It highlights the importance of judicious antibiotic usage and the need to consider the potential side effects while prescribing antibiotics. Always consider other possible causes of conjunctivitis, including viral infections, and perform a thorough patient history and examination to confirm the diagnosis.