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Psoriatic Arthritis & Reiter’s | MRCP Part 1

TL;DR

Rheum: Psoriatic Arthritis & Reiter’s Syndrome are core seronegative spondyloarthropathies tested in MRCP Part 1 via pattern recognition. Psoriatic arthritis is suggested by DIP involvement, nail changes and dactylitis, whereas reactive arthritis follows a recent GU/GI infection with asymmetric oligoarthritis, conjunctivitis and urethritis. Focus on distinguishing features, imaging clues, and first-line management. Avoid traps such as misclassifying DIP disease as rheumatoid arthritis or missing the infective trigger.


Why this matters

Questions on seronegative spondyloarthropathies (SpA) are frequent in MRCP Part 1 because they test efficient clinical reasoning: identify patterns, link associations, and choose appropriate investigations or management. Psoriatic arthritis (PsA) and reactive arthritis (Reiter’s syndrome) are especially high yield, with distinctive exam clues that often appear in single best answer (SBA) questions.

If you’re building a structured revision plan, begin with the MRCP Part 1 overview and reinforce with Free MRCP MCQs or a timed Start a mock test.


Core sections

1) Big picture: seronegative spondyloarthropathies

  • RF and anti-CCP typically negative

  • Associated with HLA-B27

  • Common features: enthesitis, dactylitis, axial involvement, uveitis

  • Includes: ankylosing spondylitis, PsA, reactive arthritis, IBD-associated arthritis

2) Psoriatic arthritis (PsA): exam essentials

Five patterns tested

  1. Asymmetric oligoarthritis

  2. DIP predominant arthritis

  3. Symmetric polyarthritis (RA mimic)

  4. Arthritis mutilans (severe, destructive)

  5. Axial disease (sacroiliitis/spondylitis)

Key clinical clues

  • Skin psoriasis: scalp, extensor surfaces

  • Nail changes: pitting, onycholysis

  • Dactylitis: “sausage digits”

  • Enthesitis: Achilles tendon, plantar fascia

Investigations

  • RF/anti-CCP usually negative

  • Raised ESR/CRP

  • X-ray: erosions + new bone formation

  • Classic late feature: “pencil-in-cup” deformity

Management

  • NSAIDs → methotrexate (peripheral disease)

  • Biologics (TNF, IL-17, IL-23 inhibitors) for severe/refractory disease

  • Co-manage skin disease (dermatology input)

Authoritative reference:

3) Reactive arthritis (Reiter’s syndrome): exam essentials

Definition: Sterile inflammatory arthritis occurring 1–4 weeks after infection

Classic triad (may be incomplete)

  • Urethritis

  • Conjunctivitis

  • Arthritis (asymmetric, lower limbs)

Common triggers

  • GU: Chlamydia trachomatis

  • GI: Campylobacter, Salmonella, Shigella, Yersinia

Other features

  • Enthesitis (heel pain)

  • Keratoderma blennorrhagica

  • Circinate balanitis

  • HLA-B27 associated

Investigations

  • Evidence of prior infection (NAAT, stool culture)

  • Raised inflammatory markers

  • Synovial fluid: sterile

Management

  • NSAIDs first line

  • Treat underlying infection (e.g., chlamydia)

  • Persistent disease: steroids or DMARDs

Authoritative references:

4) PsA vs Reactive arthritis — high-yield comparison

Feature

Psoriatic Arthritis

Reactive Arthritis

Trigger

Psoriasis

Recent infection

Pattern

DIP, oligo, axial

Oligo (lower limb)

Nails

Pitting, onycholysis

Absent

Dactylitis

Common

Possible

Extra features

Skin + nail disease

Conjunctivitis, urethritis

HLA-B27

Variable

Often positive

Imaging

Erosions + new bone

Often normal early

Treatment

NSAIDs → MTX/biologics

NSAIDs ± treat infection

5) The 5 most tested subtopics

  1. DIP involvement → PsA (not RA)

  2. Nail changes as diagnostic clue

  3. Post-infectious timeline in reactive arthritis

  4. HLA-B27 association and uveitis

  5. Radiographic hallmark: pencil-in-cup

6) 10 high-yield takeaways

  1. PsA commonly affects DIP joints

  2. Nail pitting strongly suggests PsA

  3. Dactylitis is a hallmark of SpA

  4. Reactive arthritis follows infection (1–4 weeks)

  5. Triad: urethritis + conjunctivitis + arthritis

  6. Both conditions are seronegative

  7. Enthesitis is a key differentiator from RA

  8. PsA shows new bone formation

  9. Always ask about recent diarrhoea or dysuria

  10. NSAIDs are first-line in both

Medical student revising rheumatology topics for MRCP Part 1 using notes and practice questions

Practical examples / mini-cases

MCQA 30-year-old man presents with right knee swelling and left ankle pain. He had diarrhoea 3 weeks ago. Examination reveals conjunctivitis and Achilles tendon tenderness. RF is negative.

Diagnosis? A. Rheumatoid arthritisB. GoutC. Psoriatic arthritisD. Reactive arthritisE. Septic arthritis

Answer: D. Reactive arthritis

Explanation: The preceding infection, asymmetric oligoarthritis, conjunctivitis, and enthesitis strongly indicate reactive arthritis. Septic arthritis is usually monoarticular with systemic features.


Common pitfalls (5 bullets)

  • Mislabeling DIP arthritis as RA

  • Missing preceding infection history

  • Ignoring enthesitis

  • Over-relying on RF status

  • Forgetting to treat underlying infection


Practical study-tip checklist

  • ☐ Memorise SpA patterns

  • ☐ Focus on DIP vs MCP differences

  • ☐ Revise extra-articular features

  • ☐ Practise questions via Free MRCP MCQs

  • ☐ Attempt weekly Start a mock test

  • ☐ Review axial disease concepts

Suggested reading: Spondyloarthropathies overview → /blog/spondyloarthropathies-mrcp/


FAQs

1) How to differentiate PsA from RA?

PsA involves DIP joints and nail changes, whereas RA typically spares DIP joints and is seropositive.

2) What triggers reactive arthritis?

Usually GU (chlamydia) or GI infections such as salmonella or shigella.

3) Is HLA-B27 necessary for diagnosis?

No, but it supports diagnosis and predicts severity.

4) First-line treatment?

NSAIDs for both; escalate as needed.

5) Key imaging sign in PsA?

“Pencil-in-cup” deformity on X-ray.


Ready to start?

Master these patterns through structured revision using the MRCP Part 1 overview, practise regularly with Free MRCP MCQs, and simulate exam conditions via a Start a mock test.


Sources

 
 
 

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