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Textbook/Dermatology/Primary and secondary lesion morphology
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Primary and secondary lesion morphology

Describe skin lesions precisely using reproducible primary and secondary morphology, recognise changes caused by evolution or manipulation, and communicate findings without relying on colour alone.

22 min full readUK scopeSources checked 27 Aug 2026Clinical review pending
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Time-critical presentation

Escalate urgently when assessment for Primary and secondary lesion morphology reveals systemic toxicity, airway or eye involvement, extensive skin failure, a non-blanching eruption in an unwell patient, or a rapidly changing lesion suspicious for aggressive malignancy.

Open the sections you need. The overview is shown first.
01Purpose and principlesWhat the assessment is for and the core concepts behind it.

Begin with the lesion that best preserves the untreated process. State whether it is flat, raised, fluid-filled or a loss of surface; measure it; then describe border, surface, consistency and colour. A plaque is a palpable plateau, whereas a patch is flat. A nodule is a deeper solid lesion and cannot be defined by diameter alone.

Evolution converts primary morphology into secondary change. Scratching creates excoriations and lichenification, vesicles become erosions or crust, and inflammation may leave pigment alteration. Look at multiple sites, including a fresh edge, because one old or treated lesion can misclassify the entire eruption.

Primary and secondary lesion morphology should answer a defined clinical question and be integrated with history, examination and the consequences of error. Explain uncertainty, record the sampling or observation conditions, and arrange a result-review plan rather than treating an isolated finding as self-interpreting.

Key points

  • Primary lesions include macules, patches, papules, plaques, nodules, vesicles, bullae, pustules, wheals and cysts; measure rather than estimating their size.
  • Secondary change includes scale, crust, erosion, ulceration, excoriation, fissure, atrophy, lichenification and scar, and may obscure the initiating lesion.
  • Describe colour with morphology and palpation: erythema may appear bright red, violaceous, dusky, brown or grey depending on pigmentation and inflammation depth.
  • A vesicle contains clear fluid and is usually under 1 cm; a bulla is larger, while a pustule contains visibly purulent material that is not necessarily infectious.
  • For Primary and secondary lesion morphology, describe what is seen before assigning a diagnosis, and record site, extent, symptoms, duration and change over time.
  • A technically adequate result in Primary and secondary lesion morphology can still be misleading when the wrong lesion, site, preparation or clinical question was selected.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Flat colour change

Macules and patches alter colour without palpable elevation; palpate across the boundary because subtle plaques can appear flat in photographs.

Solid raised lesions

Papules, plaques and nodules differ in depth and geometry; record size, surface, firmness, mobility and relationship to dermis or subcutis.

Fluid-filled lesions

Vesicles and bullae contain clear fluid, pustules contain cloudy material, and tense versus flaccid roofs help localise the cleavage plane.

Surface lossRed flag

Erosion is epidermal loss and usually heals without scar; ulceration extends into dermis and demands a cause, depth and edge description.

Secondary response

Scale, crust, fissuring, lichenification, atrophy and scarring record disease evolution, treatment or repeated trauma rather than a separate diagnosis.

Red flags requiring action

  • A rapidly spreading painful eruption, dusky skin, blistering, mucosal erosion or systemic illness requires urgent assessment rather than prolonged descriptive refinement.
03Method and interpretationA systematic approach to the test and its findings.
Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

  1. 01
    Inspection under diffuse lightFirst step
    Why
    Define the dominant primary lesion and surface change.
    Interpretation and limitations
    Use daylight-equivalent illumination where possible and examine more than one lesion; shadow, glare and image processing distort elevation and colour.
  2. 02
    Palpation and diascopy
    Why
    Test depth, consistency, temperature, tenderness and blanching.
    Interpretation and limitations
    Diascopy may distinguish blood within vessels from non-blanching blood or pigment, but pressure alone cannot exclude serious infection in an unwell patient.
  3. 03
    Measurement and anatomical record
    Why
    Create a reproducible baseline for change.
    Interpretation and limitations
    Use millimetres or centimetres, describe the precise site and measure the same axis at review; subjective labels such as small are unreliable.
  4. 04
    Targeted dermoscopy or sampling
    Why
    Clarify a remaining morphological differential.
    Interpretation and limitations
    Choose dermoscopy, scraping, swab or biopsy according to the question; each adds information but none repairs an unrepresentative target.
04Clinical next stepsHow the result changes management or prompts escalation.
01Planned assessmentUse structured morphological description systematicallyFirst stepThe patient is stable and the result will alter diagnosis, referral or follow-up.
  1. 1Define the question for Primary and secondary lesion morphology, explain the process and obtain valid consent before exposing, touching, photographing or sampling skin.
  2. 2Choose representative anatomy, optimise lighting or specimen technique, and document site, morphology, symptoms and relevant previous treatment.
  3. 3Interpret the result beside the full clinical pattern, communicate uncertainty and arrange ownership of results and safety-netting.
02Uncertain resultResolve discordance in Primary and secondary lesion morphologyThe technical finding conflicts with the history, lesion evolution or wider examination.
  1. 1Recheck identity, site, timing, preparation, treatment exposure and whether the selected target was genuinely representative.
  2. 2Repeat or select a complementary test only when it can distinguish the remaining important alternatives.
  3. 3Seek dermatology, pathology, microbiology or allergy advice when clinicopathological disagreement would change urgent care.
03Urgent patternDo not let structured morphological description delay escalationEscalationA rapidly progressive eruption, systemic illness, threatened vision or airway, or suspected aggressive malignancy is present.
  1. 1Stabilise immediate physiological threats and obtain same-day senior or specialty assessment according to the dominant emergency.
  2. 2DefinitiveTake time-critical images or specimens only when doing so will not postpone resuscitation, antimicrobials or definitive referral.
  3. 3Record evolution and communicate the differential, outstanding results and explicit deterioration triggers during handover.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
  • Record who will review the result arising from Primary and secondary lesion morphology, the expected timescale and the action threshold before the patient leaves.
  • Compare subsequent findings with the original site description, dimensions, symptoms and image or specimen identifiers for Primary and secondary lesion morphology.
  • Reassess earlier if rapid growth, bleeding, ulceration, fever, mucosal disease, eye symptoms or functional compromise develops.
  • Document technical limitations and previous treatment that could alter sensitivity or specificity of structured morphological description.
  • Close the loop after specialist or laboratory review, including clinicopathological disagreement and any need for repeat sampling.
06Special situationsVariants, exceptions and circumstances that change the usual approach.

Palpation changes language

A lesion that looks like a patch may become a thin plaque once elevation and induration are felt at the edge.

Pus is not infection

Sterile neutrophilic disorders can form pustules, so culture and context matter more than fluid colour alone.

Colour descriptors travel poorly

Name red, purple, brown, grey or black while also documenting warmth, blanching, scale and symptoms.

Edges hold information

An active advancing border often preserves diagnostic morphology after the centre has cleared, crusted or been treated.

One patient has several ages

Examining new, mature and resolving lesions can reconstruct evolution better than any single snapshot.

07Common pitfallsFrequent interpretation and management errors.
  1. 01

    Calling every raised lesion a rash rather than identifying its primary morphology and depth.

  2. 02

    Mistaking post-inflammatory pigment change for active erythema when warmth, scale and symptoms have settled.

  3. 03

    Describing only the most excoriated lesion and missing intact vesicles or a characteristic advancing edge.

  4. 04

    Using blanching as reassurance despite fever, shock, pain or rapidly progressive purpura.

  5. 05

    Equating all cloudy fluid with bacterial infection and prescribing without culture or clinical context.

Practice

Two practice questions

Question 1 of 20 correct
DermatologyOriginal SBA

Erosion or ulcer

A patient has a shallow moist area after a blister roof detached. The defect is confined to epidermis and no dermis is visible. Which morphological term is most accurate?

Sources and review status3 sources · checked 27 Aug 2026 · clinical review pending
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.

Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom