Anterior Segment and Cornea
Core concepts
Master the layered anatomy of the cornea and how each layer maps to disease: epithelium (recurrent erosion, epithelial basement membrane dystrophy), stroma (scarring, keratoconus, dystrophies), Descemet membrane (posterior polymorphous, Fuchs), and endothelium (pump failure, guttae). Know the corneal dystrophy classification by layer and inheritance, the difference between degeneration (bilateral, non-familial, limbal) and dystrophy (often inherited, central), and the infectious keratitis decision tree: bacterial (contact lens, purulent), fungal (agricultural trauma, feathery margins, satellite lesions), acanthamoeba (contact lens, ring infiltrate, severe pain out of proportion), and HSV (dendrite with terminal bulbs, reduced sensation).
Traps
Questions love to test the herpetic dendrite vs. healing abrasion distinction (true dendrite has terminal bulbs and stains with rose bengal; a pseudodendrite of acanthamoeba or healing epithelium does not). Another trap is confusing map-dot-fingerprint dystrophy with Fuchs - both cause recurrent erosion but only Fuchs has endothelial guttae and progressive edema. Know when penetrating keratoplasty vs. DSAEK/DMEK is preferred (endothelial disease favors lamellar).
Worked example
A contact lens wearer presents with a ring-shaped stromal infiltrate, disproportionate pain, and a dendritiform epithelial pattern that does not respond to antivirals. Rule of thumb: contact lens + ring infiltrate + severe pain = acanthamoeba until proven otherwise; confirm with confocal microscopy or culture on non-nutrient agar with E. coli, and treat with a biguanide plus diamidine (polyhexamethylene biguanide + propamidines).