Skin Sciences
Skin science questions ask you to predict what a product or treatment will do to the barrier. The high-yield concepts are pH and the acid mantle, transepidermal water loss, natural moisturizing factor, desquamation, melanogenesis, and photoaging.
Barrier and pH
Healthy skin surface pH is roughly 4.5–5.5. The acid mantle, built from sebum, sweat, and fatty acids, keeps enzymes for desquamation working and suppresses bacteria. Barrier lipids - ceramides, cholesterol, and fatty acids - form the mortar between corneocytes. When the barrier fails, transepidermal water loss rises, skin feels tight and reacts, and the client becomes a poor candidate for aggressive exfoliation.
Turnover and pigment
Keratinocytes move from the basale to the surface over roughly four weeks; this slows with age. Melanocytes transfer melanin in packets called melanosomes to surrounding keratinocytes. The Fitzpatrick scale I–VI predicts sunburn and pigment response, not skin oiliness - a common mixing up.
Aging
Intrinsic aging thins the epidermis, reduces collagen and elastin, and slows turnover. Extrinsic aging - UV, smoking, pollution - produces coarse wrinkles, mottling, and actinic change. UVA penetrates the dermis and drives photoaging; UVB drives burning. Free radicals from UV damage break down collagen; antioxidants limit that damage but do not replace sunscreen.
Worked example
A client on a retinoid wants a high-concentration glycolic peel. Retinoids thin the stratum corneum and increase photosensitivity; pairing them with a strong acid raises the risk of burning and post-inflammatory hyperpigmentation. The answer: discontinue or defer, reassess barrier health, and start with a lower-strength, higher-pH exfoliant.
Traps
Equating oily with hydrated. Assuming a high percentage of acid matters more than its pH - the pH determines how much free acid is active. And treating the Fitzpatrick scale as a skin-type system for product selection.