Seronegative inflammatory arthritis associated with psoriasis; classic features include dactylitis, enthesitis, and DIP involvement.
Also known as: PsA, psoriatic arthritis
Overview
Inflammatory arthritis occurring in patients with cutaneous or nail psoriasis. A heterogeneous disease that can affect peripheral joints (including DIPs), entheses, the axial skeleton, and digits (dactylitis), often in asymmetric patterns.
Epidemiology
Develops in 20-30% of patients with psoriasis. Skin disease usually precedes joint disease by years, but arthritis can appear first in 10-15%. Onset peaks 30-50; men and women affected roughly equally (axial disease more common in men).
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Question 1MusculoskeletalMedium
A 42-year-old man presents with a 4-month history of pain and stiffness in the distal interphalangeal joints of both hands and the right knee. He reports morning stiffness lasting about 90 minutes. PMH is significant for plaque psoriasis affecting his elbows and scalp for 10 years. Exam shows dactylitis of the right second toe, pitting of several fingernails, and scaly erythematous plaques on the extensor surfaces. ESR is elevated; rheumatoid factor and anti-CCP are negative. Hand radiographs show erosive changes at the DIP joints. Which of the following is the most appropriate first-line pharmacologic therapy?
AAdalimumab
BHydroxychloroquine
CAcitretin
DMethotrexate
EPrednisone
Reveal answer & full explanation
Correct answer: D — Methotrexate
AAdalimumab
BHydroxychloroquine
CAcitretin
DMethotrexate✓
EPrednisone
Why methotrexate is correct
This patient has psoriatic arthritis (PsA) with peripheral joint involvement, dactylitis, nail pitting, and concurrent plaque psoriasis.
For peripheral PsA with moderate-to-severe disease activity, conventional synthetic DMARDs (csDMARDs) are first-line.
Methotrexate is the most commonly used initial agent because it treats both the joint disease and the cutaneous psoriasis, is oral, and is inexpensive.
Typically dosed at 15–25 mg weekly with folic acid supplementation.
GRAPPA and EULAR guidelines support methotrexate as a preferred initial conventional synthetic DMARD in peripheral PsA with clinically relevant skin involvement, with a biologic escalated to after inadequate response to a csDMARD.
Why the others are wrong
Adalimumab — a TNF inhibitor that is highly effective in PsA but is generally reserved for patients who fail or cannot tolerate a conventional DMARD, or for those with axial disease, severe enthesitis, or rapidly erosive disease; not first-line here.
Prednisone — systemic glucocorticoids are generally avoided in PsA because tapering can precipitate a severe pustular or erythrodermic psoriasis flare; used only sparingly for short bridging.
Hydroxychloroquine — used in rheumatoid arthritis and lupus but is relatively contraindicated in psoriasis because it can trigger psoriatic flares and erythroderma.
Acitretin — an oral retinoid used for severe plaque and pustular psoriasis, so it is tempting when skin disease is prominent, but it is not a disease-modifying agent for the arthritis, does not prevent erosive joint damage, and is not recommended for peripheral PsA.
Question 2MusculoskeletalMedium
A 55-year-old man with a long history of plaque psoriasis develops pain and swelling in several finger and toe joints. Examination shows an asymmetric oligoarthritis with involvement of the distal interphalangeal joints and a diffusely swollen 'sausage' digit. Radiographs of the hands demonstrate a pencil-in-cup deformity, and rheumatoid factor is negative. Which of the following is the most likely diagnosis?
AErosive osteoarthritis
BRheumatoid arthritis
CReactive arthritis
DGouty arthropathy
EPsoriatic arthritis
Reveal answer & full explanation
Correct answer: E — Psoriatic arthritis
AErosive osteoarthritis
BRheumatoid arthritis
CReactive arthritis
DGouty arthropathy
EPsoriatic arthritis✓
Why Psoriatic arthritis is correct
Psoriatic arthritis classically occurs in a patient with cutaneous psoriasis and produces an asymmetric oligoarthritis with distal interphalangeal (DIP) involvement, dactylitis (the 'sausage' digit), and enthesitis.
The pencil-in-cup deformity is its characteristic erosive radiographic finding, and rheumatoid factor is typically negative (a seronegative spondyloarthropathy).
The combination of psoriasis, DIP disease, dactylitis, pencil-in-cup change, and negative rheumatoid factor distinguishes it from the other inflammatory arthritides.
Why the others are wrong
Gouty arthropathy — gout produces 'rat-bite' or overhanging erosions and tophi rather than the pencil-in-cup deformity, and typically begins as acute monoarticular podagra (confused-with erosive pattern).
Rheumatoid arthritis — RA is symmetric, characteristically spares the DIP joints, and is usually rheumatoid factor positive, the opposite of this asymmetric, DIP-predominant, seronegative picture (premature closure on the most common inflammatory arthritis).
Reactive arthritis — reactive arthritis follows a genitourinary or gastrointestinal infection and lacks psoriasis and the pencil-in-cup deformity (buzzword-matching on oligoarthritis).
Erosive osteoarthritis — erosive (inflammatory) hand osteoarthritis also targets the DIP joints in middle-aged adults and produces erosions, but its central erosions create a 'gull-wing' appearance rather than pencil-in-cup, and it does not cause dactylitis or associate with psoriasis (confused-with DIP-predominant erosive disease).
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Cutaneous psoriasis (especially severe or with nail involvement)
Family history of psoriasis or psoriatic arthritis
HLA-B27 (axial PsA)
Obesity (worsens disease and reduces drug response)
Smoking
Trauma (Koebner phenomenon in joints — deep Koebner)
Pathophysiology
Combined synovitis and enthesitis driven by TNF-alpha, IL-17, and IL-23 pathways. Genetic susceptibility (HLA-B27, HLA-Cw6, IL-23R variants) plus environmental triggers produce inflammation at synovium, entheses, nails, and skin.
Clinical presentation
Symptoms
Joint pain with morning stiffness >30 minutes, improving with activity
Asymmetric oligoarthritis (most common pattern), DIP-predominant, polyarticular RA-like pattern, arthritis mutilans, or axial pattern
Dactylitis — sausage digit from combined tenosynovitis and arthritis
Skin and nail findings — plaque psoriasis (look for hidden sites: scalp, umbilicus, gluteal cleft, behind ears) and nail pitting, onycholysis, oil drop sign
Signs / physical exam
Asymmetric synovitis often involving DIPs
Dactylitis — entire digit swollen, tender
Enthesitis at Achilles insertion, plantar fascia, patellar insertion
Nail changes: pitting, onycholysis, subungual hyperkeratosis
Skin plaques
Reduced spinal mobility if axial involvement
Differential diagnosis
Rheumatoid arthritis — Symmetric MCP/PIP/wrist synovitis, positive RF/CCP, no DIP unless OA also present; no enthesitis or dactylitis
Osteoarthritis (especially erosive hand OA) — DIP involvement with Heberden nodes, mechanical pattern, normal CRP
Gout — Acute monoarticular flares, MSU crystals, can mimic PsA in feet
Ankylosing spondylitis — Pure axial disease without psoriasis or skin findings
IBD-associated arthritis — Coexisting Crohn or UC; peripheral or axial pattern
Diagnostic workup
Diagnostic criteria
CASPAR classification criteria: inflammatory articular disease (joint, spine, or entheseal) plus ≥3 points: current psoriasis (2 points) or personal/family history of psoriasis (1), psoriatic nail dystrophy (1), negative RF (1), current or past dactylitis (1), and juxta-articular new bone formation on radiograph (1).
Labs
RF and anti-CCP — typically NEGATIVE (some patients have low-titer positivity)
ESR, CRP — variably elevated; can be normal in active disease
HLA-B27 — supportive in axial disease
Uric acid — exclude gout in monoarticular flare
Imaging
Plain radiographs — 'pencil-in-cup' deformity at DIPs, periostitis, fluffy new bone formation, marginal erosions, ankylosis, asymmetric sacroiliitis, non-marginal syndesmophytes (large, asymmetric, parasyndesmophytes)
Ultrasound or MRI — detects enthesitis, synovitis, dactylitis, and bone marrow edema before radiographic change
Diagnostic algorithm
Feature
RA
PsA
Symmetry
Symmetric
Often asymmetric
DIP involvement
Rare
Common
Dactylitis
No
Yes (hallmark)
Enthesitis
No
Yes (hallmark)
RF / anti-CCP
Positive 70-80%
Typically negative
Nail changes
No
Pitting, onycholysis
Radiograph
Marginal erosions, periarticular osteopenia
Pencil-in-cup, periostitis, new bone
Axial disease
Cervical only (C1-C2)
Sacroiliitis (often asymmetric), syndesmophytes
Differentiating RA from PsA — the high-yield boards comparison.
Treatment
First-line
Mild peripheral disease (oligoarthritis without poor prognostic factors): NSAIDs — naproxen, ibuprofen, meloxicam — plus intra-articular steroid injections
Conventional DMARDs for peripheral arthritis: methotrexate, sulfasalazine, leflunomide (also helps skin disease, especially MTX) — start promptly for polyarthritis, or for mono/oligoarthritis with poor prognostic factors (structural damage, elevated ESR/CRP, dactylitis, nail involvement), with MTX preferred when skin disease is significant; csDMARDs do not work for axial disease or isolated enthesitis, which move to a biologic (or JAK inhibitor) after NSAIDs
Treat skin disease in parallel with topical agents and phototherapy as needed
Second-line / adjunct
TNF inhibitors — etanercept, adalimumab, infliximab, golimumab, certolizumab — broadly effective for skin, joint, axial, and entheseal disease
IL-17 inhibitors — secukinumab, ixekizumab — particularly effective for skin and enthesitis (avoid in IBD)
IL-23 inhibitors — guselkumab, risankizumab — emerging first-line biologic for skin and peripheral joints; not recommended for axial disease (IL-23 blockade failed in axSpA trials)
IL-12/23 inhibitor — ustekinumab (skin and peripheral joints; not for axial disease)
Increased cardiovascular mortality independent of traditional risk factors
PANCE pearls
Look for psoriasis in hidden sites — scalp, umbilicus, gluteal cleft, behind the ears — before excluding PsA in a patient with inflammatory arthritis.
Dactylitis and enthesitis are highly characteristic; DIP involvement with nail pitting strongly suggests PsA over RA.
Pencil-in-cup deformity is the classic radiographic finding.
Systemic steroid tapers can trigger pustular psoriasis flares — avoid when possible.
Treat-to-target with regular assessment of joint, skin, and patient-reported outcomes is now standard.
References
ACR/NPF 2018 — 2018 ACR/NPF Guideline for the Treatment of Psoriatic Arthritis (Singh et al., Arthritis Rheumatol 2019)
GRAPPA 2021 — GRAPPA Treatment Recommendations for Psoriatic Arthritis (Coates et al., Nat Rev Rheumatol 2022)
CASPAR — Classification Criteria for Psoriatic Arthritis (Taylor et al., Arthritis Rheum 2006)
EULAR 2023 — EULAR Recommendations for the Management of Psoriatic Arthritis with Pharmacological Therapies: 2023 Update (Gossec et al., Ann Rheum Dis 2024;83(6):706-719)
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