Anatomy, Physiology and Skin/Nail Disorders
What this domain tests
Anatomy and disorder items ask where nail and skin structures are, what they do, how fast nails grow, and - critically - which findings mean "refer" rather than "cover with product."
The nail unit
The nail plate is the visible keratin structure resting on the nail bed. The matrix, hidden under the proximal nail fold, produces the plate and determines its thickness and shape; the lunula is the visible front edge of the matrix, not the matrix itself. The eponychium is the living skin at the base of the plate, while the cuticle is the dead seal on the plate; the hyponychium is the seal under the free edge. Lateral folds frame the sides and form the nail groove. Fingernails grow roughly 3 mm a month and toenails about 1 mm, so a fingernail replaces fully in about four to six months and a toenail in nine to twelve.
Skin and support structures
The epidermis is the outer layer, with a basal layer producing new cells and melanocytes supplying pigment; the dermis holds nerves, vessels, and collagen; subcutaneous fat cushions. Beneath the hand lie metacarpals and phalanges, with tendons, arteries, and lymphatics close to the surface.
Disorders that stop the service
Refer for onychomycosis (thick, yellow, crumbling plate), paronychia (red, swollen fold, possibly pus), green-black discoloration, a dark streak in the plate, sudden Beau's lines, or koilonychia (spoon nails, often iron deficiency). Never apply product over a suspected infection.
Worked example
A client's nail has lifted from the bed with a green tint after a set of acrylics. Reasoning: green-black suggests bacterial or yeast involvement beneath lifted product - a condition to be diagnosed, not cleaned up and covered. Remove the product as gently as possible, refer to a physician, and disinfect every implement used.
Traps
Calling the lunula the matrix, reversing fingernail and toenail growth timelines, and treating a dark streak as polish staining.