Anterior Segment and Cornea
Core concepts
Master the layers of the cornea (epithelium, Bowman, stroma, Descemet, endothelium) and map each dystrophy or ulcer to its layer. Know endothelial pump physiology: the endothelium does not regenerate, so any endothelial loss is permanent and predisposes to edema. Be fluent in microbial keratitis risk factors (contact lens wear, trauma, topical steroid use) and the classic signs that separate bacterial (purulent, hypopyon), fungal (feathery margins, satellite lesions), and acanthamoeba (ring infiltrate, severe pain out of proportion) disease. Understand anterior uveitis grading, the difference between granulomatous (mutton-fat, Koeppe/Busacca) and non-granulomatous findings, and the urgency of hypopyon or decreased vision.
Traps
Questions love to blur steroid indications: steroids treat inflammation but worsen active infection, so herpetic epithelial keratitis is a classic wrong answer. Another trap is confusing a dendritic ulcer (HSV epithelial) with a geographic ulcer (larger, often steroid-modified). Also watch for pterygium versus pinguecula-only pterygium crosses the limbus onto the cornea. Dry eye questions may test that tear film instability precedes corneal staining in severity.
Worked example
A contact lens wearer presents with a ring-shaped corneal infiltrate and severe pain. The ring and disproportionate pain point to acanthamoeba; culture on non-nutrient agar with E. coli overlay is the confirmatory step, and a corneal scrape is preferred over swab. Rule of thumb: any contact lens keratitis with a ring or poor response to antibacterial therapy should raise acanthamoeba before fungus.