Anatomy, Physiology, and Disorders of the Hair and Skin
Structures to know
Skin has three layers - epidermis, dermis, and subcutaneous tissue - and hair grows from a follicle whose base (the bulb) sits over the papilla, which supplies nutrients. The sebaceous gland lubricates the hair and skin; the arrector pili muscle causes goosebumps. Know the skull bones (frontal, parietal, temporal, occipital, zygomatic, maxilla, mandible), the muscles of the face and scalp (frontalis, temporalis, occipitalis, masseter, platysma), the sensory trigeminal and motor facial nerves, and the superficial temporal and occipital arteries.
Growth and cycles
Hair grows in cycles: anagen (active growth, roughly two to seven years), catagen (short transitional phase), and telogen (resting, ending in shedding). Average growth is about half an inch per month, with normal shedding of roughly 50–100 hairs a day. Hair texture has three layers: cuticle, cortex (where pigment lives), and medulla.
Disorders: treat vs. refer
The exam tests whether you can separate noncontagious conditions from those requiring referral. Noncontagious: alopecia areata (patchy, autoimmune), androgenetic alopecia, telogen effluvium, seborrheic dermatitis, hypertrichosis (excess hair), hirsutism (excess hair in women, usually hormonal). Contagious or medically significant - refer: tinea capitis/facialis, pediculosis, furuncle, carbuncle, and any undiagnosed lesion.
Traps
- Calling alopecia areata contagious - it is not.
- Confusing hypertrichosis with hirsutism.
- Diagnosing a scalp condition instead of referring it.
- Mixing up anagen and telogen, or the sensory and motor facial nerves.
Rule of thumb
If it is red, weeping, crusted, creeping, or newly appeared, do not treat it - refer it.