Most Likely Diagnosis: Gout (Option D)
The most likely diagnosis is gout (uric acid arthritis), despite the high-normal serum uric acid level, given the patient's age, recurrent self-limited monoarticular knee arthritis, and absence of systemic features. 1, 2
Key Diagnostic Reasoning
Why Gout is Most Likely
Normal uric acid does NOT exclude gout: Serum uric acid levels are normal in approximately 10-63% of acute gout attacks because uric acid behaves as a negative acute phase reactant, temporarily decreasing during acute inflammation. 1, 3
High-normal uric acid (400 µmol/L ≈ 6.7 mg/dL) is significant: This level approaches the theoretical saturation point of 6.8 mg/dL for monosodium urate crystal formation, making gout highly plausible. 3, 4
Recurrent self-limited attacks favor crystal arthropathy: The pattern of two discrete episodes three months apart, each resolving spontaneously, is classic for gout rather than septic or reactive arthritis. 5, 6
Age and demographics: While gout typically affects older males, it can occur in 30-year-olds, particularly with metabolic risk factors. 5, 7
Why Other Diagnoses are Less Likely
Pseudogout (Calcium Pyrophosphate - Option A):
- Extremely rare at age 30; pseudogout predominantly affects elderly patients (>60 years). 8
- Typically presents with chondrocalcinosis on radiographs, particularly in knee menisci, which would be mentioned if present. 1
- The high-normal uric acid level points more toward gout than pseudogout. 5
Reactive Arthritis (Option B):
- Would typically present with systemic features (conjunctivitis, urethritis, enthesitis) or recent history of gastrointestinal/genitourinary infection, which are absent here. 5
- Less likely to have completely self-limited episodes with full resolution. 5
Septic Arthritis (Option C):
- Definitively ruled out by the patient being hemodynamically stable, afebrile, and having self-limited episodes that resolved without antibiotics. 5
- Septic arthritis requires urgent treatment and does not spontaneously resolve. 5
Critical Diagnostic Pitfall
The major pitfall here is excluding gout based on "high-normal" uric acid levels. The European League Against Rheumatism explicitly states that diagnosis should not be excluded based on normal serum uric acid if clinical manifestations are suggestive. 5, 1
Essential Next Steps
Joint aspiration during the next acute episode is mandatory to confirm the diagnosis before initiating long-term urate-lowering therapy:
Synovial fluid analysis is the gold standard, looking for needle-shaped, negatively birefringent monosodium urate crystals under polarized light microscopy. 5, 1
Gram stain and culture must still be performed even if crystals are identified, as gout and septic arthritis can coexist. 5, 1
Educate the patient to seek immediate medical attention during the next flare for joint aspiration BEFORE taking any anti-inflammatory medications, as NSAIDs can reduce inflammation and make crystal identification more difficult. 2
If joint aspiration cannot be performed, ultrasound imaging can detect the "double contour sign" or tophi, which are highly specific for gout. 2
Management Considerations
Do NOT initiate urate-lowering therapy yet: After only two attacks without crystal confirmation, treatment should focus on acute episode management with NSAIDs, colchicine, or corticosteroids. 2, 3
Assess modifiable risk factors: Screen for medications that raise uric acid (diuretics, low-dose aspirin), alcohol consumption, sugar-sweetened beverages, and purine-rich foods. 2, 6
Evaluate for comorbidities: The mildly elevated urea suggests possible renal impairment, which is associated with gout and requires assessment. 5, 2