A 6-year-old girl from rural Rajasthan presents with chronic follicular keratoconjunctivitis and pannus formation. Examination reveals Arlt's line and Hebert's pits. What is the most common causative organism of trachoma?

Explanation

## Causative Agent of Trachoma **Key Point:** Trachoma is caused exclusively by *Chlamydia trachomatis*, specifically serovars A, B, Ba, and C (serovars A–C are endemic in developing countries; Ba is a variant of B). ## Epidemiology & Clinical Context **High-Yield:** Trachoma is the leading infectious cause of blindness worldwide and remains endemic in resource-limited regions of India, Africa, and the Middle East. The disease is transmitted via direct contact, fomites, and flies—particularly in settings with poor sanitation. ## Pathognomonic Signs in Trachoma | Feature | Description | Significance | |---------|-------------|-------------| | **Arlt's line** | Horizontal linear scar on upper tarsal conjunctiva | Indicates chronic/previous trachoma | | **Hebert's pits** | Depressions along upper limbus where scarred conjunctiva has retracted | Pathognomonic for previous trachoma | | **Pannus** | Vascular infiltration of superior limbus and cornea | Leads to corneal opacity and blindness | | **Follicles** | Lymphoid aggregates on tarsal conjunctiva | Hallmark of active disease | ## Chlamydia trachomatis Serovars **Mnemonic:** **A-C = Blindness; D-K = Urogenital; L1-L3 = Lymphogranuloma** - Serovars **A–C**: Trachoma (endemic keratoconjunctivitis) - Serovars **D–K**: Inclusion conjunctivitis and urogenital infection - Serovars **L1–L3**: Lymphogranuloma venereum (systemic infection) **Clinical Pearl:** Trachoma is a chronic infection that progresses through stages (TF = trachomatous inflammation–follicular; TI = trachomatous inflammation–intense; TS = trachomatous scarring; TT = trachomatous trichiasis; CO = corneal opacity). Repeated reinfections in endemic areas drive progression to blindness. ## Why Other Organisms Are Wrong - **Neisseria gonorrhoeae**: Causes acute purulent conjunctivitis (ophthalmia neonatorum in neonates), not chronic follicular keratoconjunctivitis with scarring. - **Haemophilus influenzae**: Causes acute bacterial conjunctivitis, not trachoma. - **Streptococcus pneumoniae**: Causes acute bacterial conjunctivitis, not chronic trachomatous disease.