Psoriasis vs Lichen Planus — MRCP Part 1 Guide

TL;DR

For MRCP Part 1, distinguishing psoriasis from lichen planus is a classic dermatology task. Psoriasis typically presents with well-demarcated erythematous plaques with silvery scale, whereas lichen planus produces violaceous, pruritic, polygonal papules often showing Wickham’s striae. Exam questions frequently hinge on lesion morphology, distribution, nail findings, and systemic associations.

Why this matters

Dermatology questions in MRCP Part 1 commonly test pattern recognition. Psoriasis and lichen planus are among the most frequently compared papulosquamous conditions in exam questions.

Candidates are often asked to identify the diagnosis from a short vignette describing:

• Lesion colour

• Location on the body

• Associated systemic conditions

• Nail or mucosal involvement

Recognising these distinguishing clues can quickly convert a difficult-looking MCQ into an easy mark.

For a broader overview of exam preparation and topic coverage, review the MRCP Part 1 overview.

Core clinical comparison

Psoriasis vs Lichen Planus (High-yield table)

Feature	Psoriasis	Lichen Planus
Lesion morphology	Plaques	Papules
Colour	Erythematous	Violaceous
Surface	Silvery scale	Smooth/shiny
Classic sign	Auspitz sign	Wickham’s striae
Distribution	Extensor surfaces	Flexor surfaces
Nail involvement	Pitting, onycholysis	Ridging, pterygium
Mucosal involvement	Rare	Common
Associations	Psoriatic arthritis	Hepatitis C

This table represents the most exam-relevant contrasts between the two diseases.

The five most tested subtopics

1. Lesion morphology

Psoriasis produces well-demarcated plaques covered by thick silvery scale.

Lichen planus produces small, flat-topped papules that are typically:

• Violaceous

• Polygonal

• Intensely pruritic

The classic descriptive phrase for exam answers is:

“Violaceous, pruritic, polygonal papules.”

2. Distribution pattern

Distribution patterns are extremely important in exam stems.

Psoriasis distribution

• Extensor surfaces (elbows, knees)

• Scalp

• Sacral region

• Umbilical area

Lichen planus distribution

• Flexor wrists

• Ankles

• Lower back

• Oral mucosa

A helpful memory trick:

Psoriasis = extensor diseaseLichen planus = flexor disease

3. Nail findings

Nail changes often provide the decisive diagnostic clue.

Psoriasis nail findings

• Nail pitting

• Onycholysis

• Subungual hyperkeratosis

• “Oil drop” discoloration

Lichen planus nail findings

• Longitudinal ridging

• Nail thinning

• Pterygium formation

If an MRCP question includes nail pitting, psoriasis should immediately come to mind.

4. Oral and mucosal involvement

Lichen planus commonly affects mucous membranes.

Oral lichen planus presents with:

• White lace-like lines

• Buccal mucosa involvement

• Reticular pattern known as Wickham’s striae

Psoriasis rarely affects oral mucosa, making this an important diagnostic clue.

For dermatology revision sessions covering these patterns, candidates often review structured teaching via Dermatology lectures for MRCP.

5. Systemic associations

Systemic associations frequently appear in MRCP questions.

Psoriasis associations

• Psoriatic arthritis

• Metabolic syndrome

• Cardiovascular disease

• Obesity

• Inflammatory bowel disease

Lichen planus associations

• Hepatitis C infection

• Drug reactions

• Autoimmune disorders

The hepatitis C association with lichen planus is well documented in dermatology literature.

Authoritative clinical summaries are available from:

• https://www.nhs.uk/conditions/lichen-planus/

• https://dermnetnz.org/topics/lichen-planus

Koebner phenomenon

Both conditions may demonstrate the Koebner phenomenon, where lesions develop at sites of skin trauma.

Common triggers include:

• Scratching

• Surgical scars

• Pressure or friction

Because this phenomenon occurs in both psoriasis and lichen planus, it does not reliably differentiate the two diseases in exam questions.

Histological differences (occasionally tested)

Histology is less commonly tested in MRCP Part 1, but occasionally appears in advanced questions.

Psoriasis histology

• Parakeratosis

• Elongated rete ridges

• Munro microabscesses

Lichen planus histology

• Dense band-like lymphocytic infiltrate

• Basal layer degeneration

• Saw-tooth appearance of rete ridges

Further dermatology pathology references are summarised in resources such as DermNet NZ:

https://dermnetnz.org/topics/psoriasis

Treatment overview (basic exam relevance)

MRCP questions occasionally ask about first-line management.

Psoriasis treatment

First-line therapies include:

• Topical corticosteroids

• Vitamin D analogues (calcipotriol)

• Phototherapy

• Methotrexate for severe disease

Lichen planus treatment

• Potent topical corticosteroids

• Oral steroids for severe disease

• Antihistamines for pruritus

Detailed treatment guidelines can be found through the British Association of Dermatologists:

https://www.bad.org.uk/

Mini clinical case (MRCP-style)

Question

A 39-year-old woman presents with intensely itchy purple papules on the flexor surfaces of her wrists. Examination reveals white lace-like streaks inside the oral cavity.

What is the most likely diagnosis?

A. PsoriasisB. Lichen planusC. Pityriasis roseaD. Dermatitis herpetiformisE. Tinea corporis

Answer: B — Lichen planus

Explanation

Key diagnostic clues include:

• Violaceous papules

• Flexor wrist distribution

• Oral Wickham’s striae

These findings are characteristic of lichen planus.

Psoriasis instead produces erythematous plaques with silvery scale on extensor surfaces.

To practise more exam-style dermatology questions, candidates can explore Free MRCP MCQs or simulate exam conditions via Start a mock test.

Practical MRCP Part 1 study checklist

Use this checklist during dermatology revision:

1. Memorise the description “violaceous, pruritic, polygonal papules.”

2. Associate extensor plaques with psoriasis.

3. Associate flexor papules with lichen planus.

4. Remember nail pitting = psoriasis.

5. Recall Wickham’s striae = lichen planus.

6. Link psoriatic arthritis with psoriasis.

7. Link hepatitis C with lichen planus.

8. Practise identifying lesions in photographs.

9. Use timed MCQs for pattern recognition training.

Consistent question practice remains one of the most effective ways to prepare for MRCP Part 1.

Common pitfalls

Candidates frequently lose marks due to small but important mistakes:

• Confusing erythematous plaques with violaceous papules

• Forgetting oral involvement suggests lichen planus

• Missing nail pitting as a psoriasis clue

• Assuming psoriasis is always intensely itchy

• Ignoring systemic associations such as hepatitis C

Recognising these traps can improve diagnostic accuracy in exam scenarios.

FAQs

How do you differentiate psoriasis and lichen planus clinically?

Psoriasis causes erythematous plaques with silvery scale on extensor surfaces, while lichen planus presents with violaceous pruritic papules on flexor surfaces and often involves oral mucosa.

What are Wickham’s striae?

Wickham’s striae are fine white lacy streaks seen on lichen planus lesions or oral mucosa, representing areas of hypergranulosis.

Is psoriasis associated with arthritis?

Yes. Psoriatic arthritis occurs in a significant proportion of patients and may involve peripheral joints or the axial skeleton.

Which infection is linked to lichen planus?

Lichen planus has a known association with hepatitis C infection, particularly in certain geographic regions.

Are psoriasis and lichen planus both papulosquamous disorders?

Yes. Both conditions belong to the papulosquamous group of inflammatory skin diseases characterised by papules or plaques with scale.

Ready to start?

If you are preparing for MRCP Part 1, mastering dermatology comparisons like psoriasis vs lichen planus can significantly improve exam performance.

Start your preparation with:

• MRCP Part 1 overview

• Free MRCP MCQs

• Start a mock test

Regular question practice and pattern recognition remain the most reliable strategies for exam success.

Sources

• MRCP(UK) Examination Blueprint — https://www.mrcpuk.org/

• NHS Clinical Reference — https://www.nhs.uk/conditions/lichen-planus/

• DermNet NZ Dermatology Resource — https://dermnetnz.org/topics/psoriasis

• British Association of Dermatologists — https://www.bad.org.uk/