Anatomy, Physiology, and Disorders of Hair, Skin, and Scalp
Hair and follicle
Each hair has a cuticle (outer scales), a cortex (bulk, pigment, bonds), and often a medulla (core). Growth happens at the bulb, fed by the papilla; the sebaceous gland oils the hair and the arrector pili muscle makes it stand up. The three growth stages are anagen (growth), catagen (transition), and telogen (rest and shedding). Lanugo is fine fetal hair, vellus is fine body hair, and terminal hair is pigmented and coarse.
Skin and underlying structures
Epidermis, dermis, and subcutaneous layer, with sweat glands, oil glands, follicles, and nerves in the dermis. Know the facial muscles - frontalis, orbicularis oculi, masseter, temporalis - and the principle that massage follows muscle direction. Blood reaches the face and scalp through the temporal and facial arteries; the trigeminal and facial nerves serve sensation and movement.
Disorders - the tested split
Contagious: tinea (ringworm), impetigo, scabies, lice, and bacterial or fungal infections generally. Noncontagious: alopecia areata, androgenetic alopecia, telogen effluvium, hypertrichosis (excess hair, often hormonal), hirsuties (hair in a pattern atypical for the sex), monilethrix, trichorrhexis nodosa, fragilitas crinium, and pityriasis (dandruff). Suffixes help: -itis means inflammation, -osis means a condition, -rrhea means flow.
Traps
A question will describe scaling patches and ask what to do; the answer is refuse and refer no matter how mild it looks. Another will imply that shampooing cures a scalp disease - it never does.
Rule of thumb: if it is contagious or undiagnosed, the barber's job is referral plus disinfection of everything the client touched.