Morphology Lab
Dermatology has a grammar, and these are its words. Learn each term by what separates it from its nearest neighbour, that boundary is where every exam question and every misdiagnosis lives.
Macule
A flat, non-palpable colour change smaller than 1 cm.
The original uploader was Okwikikim at English Wikipedia. · Public domain Recognize it: Close your eyes and run a finger over it, you cannot feel a macule.
vs Papule: A papule is raised and palpable; a macule is flat. Size is not the difference, elevation is.
Patch
A flat colour change larger than 1 cm, a macule past the size threshold.
James Heilman, MD · CC BY-SA 3.0 Recognize it: Still perfectly flat; only the diameter separates it from a macule.
vs Plaque: A plaque is elevated with a flat top; a patch stays level with the surrounding skin.
Papule
A raised, palpable lesion smaller than 1 cm.
E van Herk · CC BY-SA 3.0 Recognize it: Elevated and discrete, the fingertip catches its edge.
vs Plaque: Grow a papule past 1 cm, or let papules coalesce with a flat top, and you have a plaque.
See it in: Acne vulgarisSee it in: Lichen planusSee it in: Scabies
Plaque
A raised, plateau-like lesion larger than 1 cm, broader than it is tall.
Marnanel · CC BY-SA 3.0 Recognize it: The flat elevated top, like a mesa, not a hill.
vs Nodule: A nodule is deeper and dome-shaped; a plaque is broad and flat-topped.
See it in: PsoriasisSee it in: Atopic dermatitisSee it in: Nummular eczemaSee it in: Lichen planus
Nodule
A solid, palpable lesion larger than 1 cm with a significant deep component.
James Heilman, MD · CC BY-SA 3.0 Recognize it: Depth, you can grasp it between two fingers; it lives in the dermis or below.
vs Papule: A nodule is the papule's larger, deeper cousin, over 1 cm and extending downward.
Tumour
A large nodule, typically over 2 cm, a descriptive size term, not a statement of malignancy.
Alborz Fallah · CC BY-SA 3.0 Recognize it: Size alone; 'tumour' describes bulk, and benign lesions can be tumours.
Vesicle
A fluid-filled blister smaller than 1 cm.
Fisle · CC BY-SA 3.0 Recognize it: Translucent, dome-shaped, and small, light passes through the fluid.
vs Bulla: Same lesion, different size: under 1 cm is a vesicle, over 1 cm is a bulla.
Bulla
A fluid-filled blister larger than 1 cm.
Milliejen · CC BY-SA 3.0 Recognize it: A tense bulla points to a deeper (subepidermal) split; a flaccid one to a superficial split.
vs Vesicle: Purely the 1 cm threshold, but tense-versus-flaccid is the clinically loaded distinction.
Pustule
A small elevated lesion containing purulent (white-yellow) fluid.
Roshu Bangal · CC BY-SA 4.0 Recognize it: The contents are opaque, pus, not clear fluid.
vs Vesicle: A vesicle's fluid is clear; a pustule's is cloudy white-yellow. Pustules are not automatically infected, sterile pustules exist.
Wheal
A transient, oedematous, smooth elevation, the hive.
James Heilman, MD · CC BY-SA 4.0 Recognize it: It moves: individual wheals resolve within 24 hours and appear elsewhere.
vs Plaque: A plaque persists in place for days to weeks; a wheal is gone from that spot within a day.
Cyst
An encapsulated sac containing fluid or semi-solid material.
Wikimedia Commons contributor · see source for licence Recognize it: Fluctuant and mobile under the skin, often with a central punctum.
Look-alikes
The classic confusions
Side by side, with the one distinction that settles each pair.
Macule Papule
A flat, non-palpable colour change smaller than 1 cm.
A raised, palpable lesion smaller than 1 cm.
Elevation, not size: if you can feel it, it is not a macule.
Patch Plaque
A flat colour change larger than 1 cm, a macule past the size threshold.
A raised, plateau-like lesion larger than 1 cm, broader than it is tall.
Both are over 1 cm; the plaque is raised with a plateau top.
Vesicle Bulla
A fluid-filled blister smaller than 1 cm.
A fluid-filled blister larger than 1 cm.
The 1 cm rule, and always note tense versus flaccid.
Erosion Ulcer
Superficial loss of epidermis only, heals without scarring.
Loss of epidermis and at least part of the dermis, heals with scarring.
Depth decides scarring: epidermis-only heals clean, dermis scars.
Petechiae Purpura
Pinpoint non-blanching haemorrhages under 3 mm.
Non-blanching discolouration from blood outside vessels, 3 mm to 1 cm.
Size bands of the same non-blanching process: <3 mm, 3 mm–1 cm, >1 cm (ecchymosis).
Scale Crust
Visible flakes of shedding stratum corneum.
Dried exudate, serum, blood, or pus, on the skin surface.
Dry keratin versus dried exudate, crust means the surface wept.
Wheal Plaque
A transient, oedematous, smooth elevation, the hive.
A raised, plateau-like lesion larger than 1 cm, broader than it is tall.
Time is the test: a wheal at a given site is gone within 24 hours.
Hypopigmentation Depigmentation
Reduced (but not absent) melanin, lighter than baseline skin.
Complete loss of melanin, chalk- or milk-white skin.
Some melanin versus none, the Wood lamp makes the call.
Pattern
Configuration, the shapes a rash draws
After naming the lesion, name the pattern the lesions make together. Geometry is often the fastest clue on the skin.
Annular
A ring: active raised border with central clearing.
Tinea corporis, the advancing scaly edge is where the organism lives.
Targetoid
Concentric zones of colour, a true target has three.
Erythema multiforme's iris lesions, favouring the palms; dusky pseudo-targets in SJS/TEN.
Linear
A straight line, often the geometry of an external cause.
Poison ivy contact dermatitis where the leaf brushed the skin.
Grouped (herpetiform)
Lesions clustered tightly together.
Herpes simplex and zoster, grouped vesicles on an erythematous base.
Serpiginous
Wandering, snake-like tracks.
Cutaneous larva migrans tracks; the scabies burrow is a miniature version.
Nummular
Coin-shaped, round discs without central clearing.
Nummular eczema, the fully filled-in coin, unlike tinea's ring.
Reticular
A net-like, lacy pattern.
Wickham striae on lichen planus papules; livedo on the legs.
Confluent
Individual lesions merging into larger sheets.
A morbilliform drug eruption becoming confluent on the trunk.
Discrete
Lesions staying separate, each with normal skin between.
Guttate psoriasis, discrete drop-like papules after streptococcal pharyngitis.
Koebnerized (isomorphic)
New lesions arising in lines of trauma, a scratch becomes the rash.
Psoriasis, lichen planus, and vitiligo all koebnerize.
Practice