Erythema Nodosum & Multiforme — MRCP Part 1

TL;DR

For MRCP Part 1, erythema nodosum and erythema multiforme are commonly tested dermatology conditions that illustrate systemic disease associations and immune-mediated skin reactions. Candidates should recognise their distinct lesion morphology, triggers, and pathophysiology. The erythema nodosum erythema multiforme MRCP Part 1 topic often appears in MCQs focusing on causes, classic lesion patterns, and associated diseases such as sarcoidosis or herpes simplex infection.

Erythema Nodosum & Erythema Multiforme — MRCP Part 1 High-Yield Guide

Dermatology questions in MRCP Part 1 frequently rely on pattern recognition and systemic associations rather than detailed dermatopathology. Two inflammatory conditions that repeatedly appear in examination questions are erythema nodosum (EN) and erythema multiforme (EM).

Although both present with erythematous skin lesions, they differ significantly in pathophysiology, distribution, triggers, and systemic associations. Understanding these differences is crucial for answering clinical scenario questions correctly.

If you are preparing for the exam, start with the full MRCP Part 1 overview and practise dermatology questions through the Free MRCP MCQs platform.

Why this matters for MRCP Part 1

Dermatology questions often test whether candidates can:

• recognise classic lesion morphology

• identify systemic disease associations

• understand immune-mediated mechanisms

• distinguish between similar appearing dermatological conditions

Both erythema nodosum and erythema multiforme are examples of hypersensitivity reactions with characteristic clinical presentations. MRCP questions frequently link them with infections, autoimmune disease, or drug exposure.

Core High-Yield Concepts

Below are the most important exam points candidates should remember.

1. Erythema Nodosum: Definition

Erythema nodosum is a septal panniculitis without vasculitis that presents as tender erythematous nodules in the subcutaneous tissue.

Typical clinical features:

• painful red nodules

• usually on anterior shins

• bilateral and symmetrical

• lesions evolve into bruise-like discoloration

Unlike many dermatological lesions, erythema nodosum involves inflammation of the subcutaneous fat rather than the epidermis.

2. Causes of Erythema Nodosum

Exam questions commonly ask for the most frequent triggers.

Important causes include:

• Streptococcal infection

• Sarcoidosis

• Tuberculosis

• Inflammatory bowel disease

• Pregnancy

• Oral contraceptive pills

For exam purposes, sarcoidosis and streptococcal infection are the most commonly tested associations.

3. Löfgren Syndrome (Classic Exam Association)

Löfgren syndrome is an acute form of sarcoidosis characterised by the triad:

1. erythema nodosum

2. bilateral hilar lymphadenopathy

3. acute arthritis

This presentation frequently appears in MRCP scenario-based questions, particularly when a chest X-ray is included in the stem.

Further information on sarcoidosis associations can be found via the British Association of Dermatologists patient guidance:https://www.bad.org.uk/pils/erythema-nodosum/

4. Erythema Multiforme: Definition

Erythema multiforme is an acute immune-mediated hypersensitivity reaction involving the skin and occasionally mucous membranes.

The hallmark feature is the target lesion, which consists of three zones:

1. central dusky or necrotic area

2. pale oedematous ring

3. outer erythematous rim

Recognition of this pattern is a high-yield visual clue for MRCP dermatology questions.

5. Most Common Cause of Erythema Multiforme

The most frequent trigger is herpes simplex virus infection.

Other possible causes include:

• Mycoplasma pneumoniae infection

• medications

• other viral infections

Clinical information about erythema multiforme can be found through DermNet NZ, an authoritative dermatology resource:https://dermnetnz.org/topics/erythema-multiforme

6. EM Minor vs EM Major

MRCP candidates should recognise the distinction between forms of erythema multiforme.

Feature	EM Minor	EM Major
Mucosal involvement	Minimal or absent	Present
Severity	Mild	Moderate
Systemic symptoms	Rare	May occur
Confusion with SJS	No	Historically confused

A key exam point is that Stevens-Johnson syndrome (SJS) is now considered a separate disorder rather than a severe form of erythema multiforme.

7. Pathophysiology Differences

Understanding the mechanism helps differentiate the two diseases.

Condition	Pathophysiology
Erythema nodosum	Delayed hypersensitivity causing septal panniculitis
Erythema multiforme	Cytotoxic immune reaction targeting keratinocytes

Thus EN involves subcutaneous fat, whereas EM primarily affects epidermal keratinocytes.

8. Lesion Distribution

Another common exam clue is lesion distribution.

Erythema nodosum

• anterior shins

• deep nodules

• painful lesions

Erythema multiforme

• hands

• feet

• extensor surfaces

• target lesions

9. Investigations

Investigations are usually aimed at identifying the underlying cause.

For erythema nodosum:

• chest X-ray (sarcoidosis, tuberculosis)

• throat swab or ASO titre (streptococcal infection)

• inflammatory markers

For erythema multiforme:

• usually a clinical diagnosis

• evaluation for HSV or Mycoplasma infection if suspected

10. Management Principles

Management focuses primarily on treating the underlying cause.

Erythema nodosum

• NSAIDs for symptom relief

• treat infection or systemic disease

• rest and supportive care

Erythema multiforme

• treat HSV infection

• supportive skin care

• antiviral prophylaxis in recurrent cases

Candidates can practise similar clinical scenarios through Free MRCP MCQs or simulate exam conditions with a Start a mock test.

Practical Mini-Case (MRCP Style)

A 30-year-old woman presents with painful erythematous nodules on both shins and mild fever. She reports ankle pain and fatigue. Chest X-ray demonstrates bilateral hilar lymphadenopathy.

What is the most likely diagnosis?

A. Erythema multiformeB. Erythema nodosumC. PsoriasisD. Urticaria

Answer: B. Erythema nodosum

Explanation

Painful nodules on the anterior shins strongly suggest erythema nodosum. When associated with bilateral hilar lymphadenopathy and arthritis, this constellation represents Löfgren syndrome, a presentation of acute sarcoidosis.

Pattern recognition of such clinical triads is a frequent testing strategy in MRCP Part 1.

Five Most Tested Subtopics

For efficient revision, focus on these areas:

1. Morphology of target lesions

2. Causes of erythema nodosum

3. Sarcoidosis association (Löfgren syndrome)

4. HSV-associated erythema multiforme

5. Differentiation from Stevens-Johnson syndrome

Practical Study-Tip Checklist

Before the exam, ensure you can quickly recall the following:

✔ Target lesions → erythema multiforme✔ Painful shin nodules → erythema nodosum✔ Sarcoidosis association → erythema nodosum✔ HSV trigger → erythema multiforme✔ Löfgren syndrome triad✔ Distribution patterns of both conditions✔ Difference between EM and SJS/TEN

Common Pitfalls

• Confusing erythema multiforme with Stevens-Johnson syndrome

• Forgetting HSV infection as the most common cause of EM

• Missing sarcoidosis association with erythema nodosum

• Misidentifying target lesions

• Assuming drugs are the main cause of EM (more typical of SJS/TEN)

FAQs

What is the most common cause of erythema nodosum?

The most common trigger is streptococcal infection, although sarcoidosis is a classic association often tested in MRCP examinations.

What infection most commonly triggers erythema multiforme?

The most common cause is herpes simplex virus infection, particularly recurrent HSV infections.

Is erythema nodosum a form of vasculitis?

No. Erythema nodosum is septal panniculitis without vasculitis, affecting the subcutaneous fat layer.

How do you recognise erythema multiforme lesions?

The hallmark feature is the target lesion, with three concentric zones and a central dusky area.

Is erythema multiforme the same as Stevens-Johnson syndrome?

No. Stevens-Johnson syndrome and toxic epidermal necrolysis are separate severe drug-related conditions, distinct from erythema multiforme.

Ready to start?

Dermatology questions in MRCP Part 1 reward strong pattern recognition and repeated practice.

Explore the complete MRCP Part 1 overview and strengthen your preparation using the Free MRCP MCQs or try a timed Start a mock test to assess your readiness before exam day.

Sources

MRCP(UK) Examination Blueprinthttps://www.mrcpuk.org/mrcpuk-examinations/part-1

British Association of Dermatologists – Erythema Nodosumhttps://www.bad.org.uk/pils/erythema-nodosum/

DermNet NZ – Erythema Multiformehttps://dermnetnz.org/topics/erythema-multiforme

Kumar & Clark’s Clinical Medicine (Elsevier)