Psoriatic arthritis (PsA) is one of those Step diagnoses that rewards pattern recognition and punishes sloppy thinking. The stem may scream “psoriasis,” but the answer choices will try to bait you into rheumatoid arthritis, gout, reactive arthritis, or even osteoarthritis. The key is learning what must be true in PsA—and what findings are subtle “tells” for the distractors.
Tag: MSK > Bone & Joint Disorders
Clinical Vignette (Q-bank style)
A 38-year-old man presents with 4 months of progressive joint pain and morning stiffness lasting about 1 hour. He reports swelling in several fingers and difficulty gripping objects. He has a 10-year history of plaque psoriasis treated intermittently with topical steroids. Exam shows scaly erythematous plaques on the extensor surfaces, pitting of several fingernails, and diffuse swelling of the right index finger (“sausage digit”). There is tenderness over the distal interphalangeal (DIP) joints and decreased range of motion. ESR is mildly elevated; rheumatoid factor is negative. X-ray of the hands shows erosive changes and new bone formation.
Question: What is the most likely diagnosis?
Correct Answer: Psoriatic Arthritis
Psoriatic arthritis is a seronegative spondyloarthropathy associated with psoriasis and characterized by inflammatory arthritis plus distinctive clinical/radiographic clues.
Why this is PsA (the “must-not-miss” clues)
- Psoriasis history (often precedes arthritis, but not always)
- Nail changes: pitting, onycholysis, “oil drop” discoloration
- Dactylitis (“sausage digits”) due to tenosynovitis + joint inflammation
- DIP involvement is classic (though PsA can involve other patterns)
- Seronegative: RF negative (and typically anti-CCP negative)
- X-ray: erosions + new bone formation
- High-yield imaging phrase: “pencil-in-cup” deformity (classically)
- Can also see periostitis and joint space narrowing
Step-friendly summary of PsA patterns (know these)
PsA can present in several patterns:
- Asymmetric oligoarthritis (common)
- DIP-predominant arthritis
- Symmetric polyarthritis (can mimic RA but with nail/DIP clues)
- Arthritis mutilans (severe destructive)
- Axial disease (sacroiliitis, inflammatory back pain)
What to Do Clinically (high yield management framework)
You’re not usually asked to choose the entire regimen, but Step questions love “next best step” logic.
- Mild peripheral disease: NSAIDs ± intra-articular steroids
- Persistent/moderate-severe: DMARDs (e.g., methotrexate, sulfasalazine, leflunomide)
- Severe, axial disease, or DMARD failure: TNF-α inhibitors (e.g., etanercept, infliximab, adalimumab)
- Also used: IL-17 inhibitors (secukinumab), IL-12/23 inhibitors (ustekinumab)
Pearl: Steroids can help symptoms but systemic steroids may trigger psoriasis flares when tapered—testable nuance.
Why Every Answer Choice Matters (Systematic Distractor Breakdown)
Below is how to “disarm” common trap diagnoses using 1–2 discriminators.
Distractor 1: Rheumatoid Arthritis (RA)
Why it’s tempting: inflammatory arthritis + morning stiffness.
Why it’s wrong here:
- RA classically affects MCP and PIP, not DIP.
- Nail pitting and dactylitis point away from RA.
- RA is associated with RF and anti-CCP positivity (not required, but a common clue).
- RA X-ray: marginal erosions and periarticular osteopenia, not new bone formation.
High-yield RA tells:
- Symmetric small-joint disease (MCP/PIP)
- Ulnar deviation, swan-neck, boutonnière deformities
- Extra-articular: rheumatoid nodules, ILD, pericarditis
Distractor 2: Osteoarthritis (OA)
Why it’s tempting: DIP involvement is common.
Why it’s wrong here:
- OA pain is worse with use, better with rest; morning stiffness is typically < 30 minutes.
- OA is noninflammatory (no prominent warmth/swelling, minimal systemic markers).
- OA has Heberden nodes (DIP) and Bouchard nodes (PIP)—bony enlargement, not sausage digits.
High-yield OA imaging:
- Joint space narrowing, osteophytes, subchondral sclerosis, subchondral cysts
Distractor 3: Gout
Why it’s tempting: acute joint pain can involve small joints, and inflammatory markers may rise.
Why it’s wrong here:
- Stem describes chronic inflammatory arthritis over months, not episodic flares.
- Key PsA clues: psoriasis + nail pitting + dactylitis + DIP pattern.
- Gout often hits 1st MTP (podagra) first; tophi in chronic disease.
High-yield gout clues:
- Synovial fluid: needle-shaped, negatively birefringent monosodium urate crystals
- Risks: thiazides, loop diuretics, lead, high purine intake, tumor lysis
Distractor 4: Reactive Arthritis (ReA)
Why it’s tempting: seronegative spondyloarthropathy with asymmetric arthritis, sometimes dactylitis-like swelling.
Why it’s wrong here:
- ReA typically follows GI or GU infection (e.g., Chlamydia, Campylobacter, Shigella, Salmonella, Yersinia).
- Classic extras: conjunctivitis/uveitis, urethritis/cervicitis, keratoderma blennorrhagicum.
- Psoriasis + nail changes are more consistent with PsA.
High-yield ReA phrase: “Can’t see, can’t pee, can’t climb a tree” (but don’t force it—look for infection trigger).
Distractor 5: Ankylosing Spondylitis (AS)
Why it’s tempting: also seronegative and can be associated with psoriasis.
Why it’s wrong here:
- This vignette is peripheral small-joint predominant (DIP, dactylitis).
- AS centers on axial symptoms: inflammatory back pain, sacroiliitis, reduced chest expansion.
High-yield AS imaging:
- X-ray: bamboo spine, sacroiliac joint sclerosis/erosions
- Strong association: HLA-B27
Distractor 6: Septic Arthritis
Why it’s tempting: swollen, painful joint and elevated inflammatory markers.
Why it’s wrong here:
- Time course is months, with multi-joint pattern and psoriasis signs.
- Septic arthritis is usually acute, monoarticular, severe pain with fever.
- Diagnosis hinges on arthrocentesis (high WBCs, organism).
High-yield septic arthritis:
- Most common: S. aureus
- Sickle cell: Salmonella
- Gonococcal: migratory polyarthralgia/tenosynovitis in young adults
One Table to Lock It In: PsA vs RA vs OA (Classic Step Comparisons)
| Feature | Psoriatic arthritis | Rheumatoid arthritis | Osteoarthritis |
|---|---|---|---|
| Joint pattern | Often DIP, asymmetric; dactylitis | MCP/PIP, symmetric | DIP/PIP, 1st CMC; weight-bearing joints |
| Morning stiffness | > 30–60 min | > 1 hour | < 30 min |
| Serology | RF−, anti-CCP− (seronegative) | RF+ and/or anti-CCP+ | Negative |
| Key clues | Psoriasis, nail pitting, pencil-in-cup | Nodules, ulnar deviation, erosions/osteopenia | Osteophytes, Heberden/Bouchard nodes |
| Path | Enthesitis + synovitis | Autoimmune synovitis (pannus) | Degenerative cartilage loss |
High-Yield PsA “Buzzwords” (that actually mean something)
- Dactylitis = diffuse digit swelling due to tenosynovitis
- Enthesitis = inflammation where tendons/ligaments insert into bone (think heel pain/Achilles)
- Nail pitting = strong psoriasis-associated clue
- Pencil-in-cup = erosions + bone remodeling/new bone formation
- Seronegative doesn’t mean “mild”—it means RF/anti-CCP aren’t driving the diagnosis
Exam-Day Takeaway
If you see psoriasis + nail pitting + DIP arthritis and/or dactylitis, your default should be psoriatic arthritis—then quickly rule out RA (MCP/PIP, anti-CCP), OA (brief stiffness, osteophytes), and gout (episodic flares, crystals). On Step, the fastest path to the right answer is matching the joint distribution + skin/nail clues + inflammatory pattern.