A 5-year-old boy is brought to the emergency department by his parents because he has been… — USMLE Step 2 CK

A $5$ -year-old boy is brought to the emergency department by his parents because he has been refusing to walk since this morning. The parents report that he had a subjective fever at home yesterday but otherwise has been healthy. His immunizations are up to date. His temperature is $38.9^\circ\text{C}$ ( $102^\circ\text{F}$ ), blood pressure is $100/60\text{ mmHg}$ , pulse is $115/\text{min}$ , and respirations are $22/\text{min}$ . Physical examination reveals a distressed child who cries when the right leg is touched. The right hip is held in a position of flexion, abduction, and external rotation. Any attempted passive range of motion of the right hip elicits severe pain. Laboratory studies show a leukocyte count of $16,500/\text{mm}^3$ and an erythrocyte sedimentation rate (ESR) of $55\text{ mm/hr}$ . Radiographs of the pelvis and bilateral hips demonstrate no fractures or structural abnormalities. Which of the following is the most appropriate next step in management?

Ultrasound-guided arthrocentesisAnswer
B
Initiation of intravenous ceftriaxone and vancomycin
C
Magnetic resonance imaging (MRI) of the right hip
D
Observation and administration of oral ibuprofen
E
Immediate open arthrotomy and joint washout

Answer

Ultrasound-guided arthrocentesis

The correct answer is ultrasound-guided arthrocentesis. This pediatric patient presents with acute fever, inability to bear weight, significantly elevated inflammatory markers (WBC and ESR), and severe pain with passive hip movement. These clinical signs fulfill all four predictors of the Kocher criteria, making septic arthritis highly probable (approximately 99% likelihood). Because septic arthritis is an orthopedic emergency that can rapidly cause irreversible cartilage damage and avascular necrosis of the femoral head, immediate definitive diagnosis is required. Ultrasound-guided arthrocentesis is the standard next step to confirm the diagnosis and obtain synovial fluid for Gram stain, cell count, and culture before initiating targeted antimicrobial therapy.

Step-by-Step Solution

Analyze the clinical presentation and vital signs.

The patient is a child with acute onset of fever (

38.9^\circ\text{C}

), inability to bear weight, and a hip held in a classic position of comfort (flexion, abduction, external rotation) to maximize joint capsule volume.

Identifying these key features helps differentiate severe pathology from benign pediatric conditions like transient synovitis.

Evaluate the laboratory values using the Kocher criteria.

The patient has a fever

>38.5^\circ\text{C}

, is non-weight-bearing, has a WBC

>12,000/\text{mm}^3

, and an ESR

>40\text{ mm/hr}

. Having all four criteria yields a

99\%

probability of septic arthritis.

The Kocher criteria stratify the risk of septic arthritis versus transient synovitis.

Determine the most appropriate immediate diagnostic intervention.

The correct action is to perform a joint aspiration (arthrocentesis) before starting any antibiotics.

Synovial fluid analysis (Gram stain, cell count, culture) is required for definitive diagnosis and to guide precise antibiotic therapy, preventing permanent ischemic necrosis of the femoral head.

Key Concept

Pediatric Septic Arthritis Diagnostics

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