Reactive arthritis is one of those Step “gotcha” diagnoses where the stem screams infection, but the joint problem is actually sterile inflammation after an infection. The fastest way to lock it in on test day is to (1) recognize the classic pattern and (2) know why the tempting distractors are wrong. Let’s do a Q-bank style breakdown where every answer choice teaches you something.
Tag
MSK > Bone & Joint Disorders
Clinical Vignette (Q-bank style)
A 24-year-old man comes to clinic for 3 days of right knee pain and swelling. He also reports burning with urination and red, irritated eyes. Two weeks ago, he had unprotected sex with a new partner. He is afebrile. Exam shows a warm, swollen right knee with limited range of motion. There is mild conjunctival injection. A small shallow painless ulcer is noted on the glans penis. Arthrocentesis reveals WBC 18,000/µL (neutrophil predominant), no organisms on Gram stain, and negative cultures.
Which of the following is the most likely diagnosis?
A. Disseminated gonococcal infection
B. Reactive arthritis
C. Septic arthritis due to Staphylococcus aureus
D. Gout
E. Rheumatoid arthritis
Correct Answer: B. Reactive arthritis
Why this is the best answer
This stem is basically the “reactive arthritis starter pack”:
- Timing: arthritis 1–4 weeks after a GI or GU infection
- Classic triad (often incomplete):
- Arthritis (typically asymmetric oligoarthritis of lower extremities: knee, ankle)
- Urethritis/cervicitis (dysuria)
- Conjunctivitis/uveitis (red eye)
- Mucocutaneous findings:
- Circinate balanitis (painless shallow genital lesions)
- Keratoderma blennorrhagicum (hyperkeratotic skin lesions on palms/soles; more board-style than real-life)
- Synovial fluid: inflammatory (often 2,000–50,000 WBC/µL), sterile (negative Gram stain/culture)
High-yield path & associations (Step 1/2)
- Triggering infections
- GU: Chlamydia trachomatis (classic)
- GI: Salmonella, Shigella, Campylobacter, Yersinia
- Genetics: associated with HLA-B27 (also ankylosing spondylitis, psoriatic arthritis, IBD-associated arthritis)
- Mechanism (board-relevant): immune-mediated arthritis after infection; joint is not directly infected
- Treatment (Step 2 management vibe):
- NSAIDs first-line
- If persistent: intra-articular steroids, DMARDs (e.g., sulfasalazine)
- Treat the triggering infection if still present (e.g., chlamydia)
- Important nuance: antibiotics do not treat the arthritis itself once it’s reactive, but do treat ongoing infection and reduce transmission
Synovial Fluid Cheat Table (test-day anchor)
| Condition | WBC (cells/µL) | Crystals | Gram stain/culture | Pattern |
|---|---|---|---|---|
| Noninflammatory (OA) | < 2,000 | None | Negative | Wear-and-tear pain, minimal warmth |
| Inflammatory (RA, reactive) | 2,000–50,000 | None | Negative | Warm, swollen joint(s) |
| Septic arthritis | often > 50,000 (can overlap) | None | Often positive | Fever, severe pain, can destroy joint fast |
| Crystal arthritis (gout/pseudogout) | 2,000–50,000 | Present | Negative | Acute flares; crystals clinch |
This vignette: inflammatory WBC count + sterile fluid + post-GU symptoms = reactive.
Now, Why Each Distractor Is Wrong (and what it would look like instead)
A. Disseminated gonococcal infection (DGI)
Why it’s tempting: young sexually active patient + joint symptoms.
Why it’s wrong here:
- DGI classically causes:
- Migratory polyarthralgia, tenosynovitis, and dermatitis (pustular/vesiculopustular lesions)
- Can progress to septic arthritis, often monoarticular
- Synovial fluid in gonococcal septic arthritis may show high WBC, and cultures can be negative, which confuses people.
Key differentiators:
- This stem has a strong reactive pattern: urethritis + conjunctivitis + sterile inflammatory fluid + mucocutaneous lesion consistent with circinate balanitis.
- DGI often has systemic symptoms (fever) and skin lesions on trunk/extremities rather than classic reactive mucosal findings.
USMLE move: If you see tenosynovitis + dermatitis + migratory arthralgias, think DGI. If you see post-infectious asymmetric oligoarthritis + conjunctivitis/uveitis, think reactive arthritis.
C. Septic arthritis due to Staphylococcus aureus
Why it’s tempting: hot swollen knee is always scary.
Why it’s wrong here:
- S. aureus septic arthritis usually presents with:
- Acute severe monoarthritis, often with fever
- Synovial WBC often very high (commonly >50,000)
- Positive Gram stain/culture (not always, but commonly)
What would push you toward septic arthritis:
- Older age, prosthetic joint, IVDU, diabetes, immunosuppression
- Bacteremia signs, rigors, marked systemic illness
- Purulent fluid; rapid joint destruction risk
High-yield pearl: Septic arthritis is a medical emergency—aspirate, culture, and start empiric antibiotics after cultures. But if the stem emphasizes post-infectious timing + sterile fluid, they’re usually steering you away from septic.
D. Gout
Why it’s tempting: acute monoarthritis with inflammation.
Why it’s wrong here:
- Gout requires monosodium urate crystals:
- Needle-shaped
- Negatively birefringent under polarized light
- Typical distribution:
- 1st MTP (podagra) is classic
- Also midfoot, ankle, knee, elbow
- Often associated with hyperuricemia risk factors (diuretics, CKD, alcohol, high purine diet), though remember: serum uric acid can be normal during an acute flare.
What would make it gout in the stem:
- Crystal findings on arthrocentesis
- Sudden onset nocturnal pain, podagra, tophi, triggers like binge drinking
No crystals + classic urethritis/conjunctivitis pattern = not gout.
E. Rheumatoid arthritis (RA)
Why it’s tempting: inflammatory arthritis with swollen joint(s).
Why it’s wrong here:
- RA is typically:
- Symmetric inflammatory polyarthritis
- Involves MCP, PIP, wrists (spares DIP)
- Morning stiffness > 1 hour
- Chronic course
- Extra-articular: nodules, ILD, pericarditis, anemia of chronic disease
- Serologies: RF, anti-CCP (anti-CCP more specific)
What would make it RA instead:
- Weeks–months of symmetric small joint pain and stiffness
- No preceding infection timing cue
- No urethritis/conjunctivitis pattern
Reactive arthritis is usually asymmetric and lower-extremity predominant.
High-Yield “Reactive Arthritis Locks” (memorize these)
- Trigger: chlamydia or GI bugs (Salmonella/Shigella/Campylobacter/Yersinia) 1–4 weeks earlier
- Pattern: asymmetric oligoarthritis of lower extremities
- Eyes + GU: conjunctivitis/uveitis + urethritis/cervicitis
- Sterile inflammatory synovial fluid
- HLA-B27 association
- Enthesitis is common (e.g., Achilles tendon pain, plantar fasciitis)
Test-Day Pitfalls (how they try to trick you)
- “Cultures are negative” does not always exclude infection (gonococcal can be culture-negative), so look for the whole pattern:
- Reactive: post-infectious timing + mucocutaneous + eye symptoms
- DGI: tenosynovitis + dermatitis + migratory arthralgia
- Don’t over-rely on the full triad. Many patients don’t have all three findings.
- Synovial WBC ranges overlap. Use context: Gram stain/culture, symptoms, and risk factors.
Mini-Recap (what you should pick in 10 seconds)
Young patient + asymmetric swollen knee + dysuria + conjunctivitis + sterile inflammatory synovial fluid after sexual exposure → Reactive arthritis.