Nail and Skin Anatomy, Physiology, and Disorders
The nail unit
Know the parts and what each does. The matrix is the factory: it produces the nail plate, and its length and shape determine plate thickness and curve. The visible lunula is only the front edge of the matrix - the matrix extends well behind it, so damage near the base can permanently distort growth. The nail bed is the pink foundation under the plate, richly supplied with nerves and blood vessels; the plate itself has neither, which is why a nail plate can be filed without pain but a nick to the bed hurts. The eponychium is the living tissue at the base that seals the nail; the cuticle is the thin dead layer on the plate that this tissue sheds. Also know the lateral folds and sidewalls, nail grooves, hyponychium, free edge, and mantle.
Growth: fingernails average about 1/8 inch (roughly 3 mm) per month and replace in about 4–6 months; toenails grow more slowly and may take 9–12 months. Growth varies with age, season, health, pregnancy, and trauma.
Disorders you must recognize and refer
Nail technicians identify and refer - never diagnose or treat. Learn the classic pictures: onychomycosis (thick, crumbly, yellow-brown), Pseudomonas (green-black discoloration under lifting product), paronychia (red, swollen, painful fold), onychocryptosis (ingrown), onycholysis (separation), onychorrhexis (ridging/splitting), koilonychia (spooning), Beau's lines (horizontal dents after illness), leukonychia (white spots), and onychophagy (bitten nails).
Trap: questions pair a disease (infectious, refer and do not service) with a disorder (structural change) and expect you to keep the distinction. Another trap is the cuticle/eponychium swap.
Rule of thumb: matrix = factory, bed = foundation, plate = product.