Skin Disorders and Esthetics Chemistry
This domain tests recognition and referral first, then the chemistry behind the products you would use. Expect lesion vocabulary, acne and rosacea basics, pigmentation disorders, allergic versus irritant reactions, and acid/peel chemistry.
Lesions and conditions
Primary lesions include macules, papules, pustules, nodules, vesicles, and wheals; secondary lesions include scales, crusts, excoriations, fissures, and scars. Acne vulgaris involves follicular plugging, sebum, and Cutibacterium acnes. Rosacea presents as flushing, telangiectasia, and papules without comedones - triggers include heat, alcohol, and spice. Melasma is hormonally and UV-driven; post-inflammatory hyperpigmentation follows injury or inflammation. Seborrheic dermatitis and eczema are referred.
Reactions
Irritant contact dermatitis is dose-dependent and burns or stings immediately. Allergic contact dermatitis is immune-mediated, may appear 24–72 hours later, and can worsen with repeat exposure - which is why patch testing and ingredient history matter.
Chemistry you must handle
pH runs 0–14; each unit is a tenfold change. AHAs (glycolic, lactic) are water-soluble keratolytics; glycolic is the smallest and penetrates fastest. BHA (salicylic acid) is lipid-soluble, so it works inside the pore and suits oily, comedonal skin; avoid or defer it with aspirin allergy or pregnancy. Emulsions are oil-in-water or water-in-oil; surfactants lift oil and debris; preservatives prevent microbial growth in water-containing products.
Worked example
An oily, comedonal client asks for an exfoliant. Reasoning: the target is inside the follicle, so a lipid-soluble BHA at a properly formulated pH fits better than a strong AHA. Start low, patch test, and pair with non-comedogenic hydration.
Traps
Calling rosacea "adult acne." Promising a peel will "cure" melasma. And forgetting that higher percentage with the wrong pH can be less effective and more irritating.