A 68-year-old man is evaluated in the hospital for sudden-onset, severe right knee pain. H… — USMLE Step 2 CK

A 68-year-old man is evaluated in the hospital for sudden-onset, severe right knee pain. He is currently on postoperative day 6 following an uncomplicated robotic-assisted prostatectomy for prostate cancer. He reports that the pain began abruptly overnight and is so severe that he cannot tolerate the weight of his bedsheet on his leg. His medical history is significant for hypertension, type 2 diabetes mellitus, and chronic kidney disease stage 3. His temperature is $38.4^\circ\text{C}$ ( $101.1^\circ\text{F}$ ), blood pressure is $138/85\text{ mmHg}$ , and pulse is $98/\text{min}$ . Physical examination reveals a markedly swollen, erythematous, and warm right knee. There is a palpable effusion, and both active and passive range of motion are severely limited by pain. The surgical incisions in the abdomen are clean, dry, and intact with no surrounding erythema. Which of the following is the most appropriate next step in management of this patient's joint pain?

Arthrocentesis and synovial fluid analysisAnswer
B
Administration of intravenous vancomycin and ceftriaxone
C
Lower extremity venous Doppler ultrasound
D
Measurement of serum uric acid concentration
E
Magnetic resonance imaging of the right knee

Answer

The most appropriate next step in management is arthrocentesis and synovial fluid analysis.

The correct response is arthrocentesis and synovial fluid analysis. An acute, hot, swollen, and exquisitely tender joint with restricted range of motion is a rheumatologic emergency. In a postoperative patient, surgical stress often triggers acute crystal arthropathies (gout or pseudogout), but septic arthritis must be definitively ruled out due to its potential to rapidly destroy joint cartilage and cause systemic sepsis. Synovial fluid analysis (checking for leukocyte count, crystal presence under polarized light, Gram stain, and culture) is the gold standard diagnostic step. It must be performed before the administration of antibiotics, as early antibiotics can quickly render synovial fluid cultures negative, severely complicating long-term management.

Step-by-Step Solution

Identify the primary clinical problem from the vignette.

The patient has an acute, atraumatic, inflammatory monoarthritis of the knee with systemic fever occurring in the postoperative period.

Recognizing the 'hot, swollen joint' is critical for prioritizing life- or limb-threatening conditions.

Formulate a differential diagnosis based on the timeline and physical exam.

The primary differentials are septic arthritis (which can rapidly destroy the joint) and acute crystal arthropathies like gout or pseudogout (which can mimic infection and cause fever, especially triggered by surgical stress).

Both conditions present identically on physical examination and require fluid analysis to distinguish.

Select the intervention that provides a definitive diagnosis without compromising future treatment.

Arthrocentesis to obtain synovial fluid for cell count, Gram stain, culture, and crystal analysis is chosen.

It definitively distinguishes between crystals and bacteria. Starting antibiotics beforehand or relying on indirect markers (like serum uric acid or imaging) will compromise patient care.

Key Concept

The initial evaluation of a single, acutely inflamed joint (monoarthritis) must prioritize ruling out septic arthritis via arthrocentesis and synovial fluid analysis before initiating empiric antibiotic therapy.

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