A 59-year-old man is evaluated in the emergency department after sustaining a puncture wound to the right foot 12 days ago. He is a construction worker. He stepped on a metallic object that punctured through his leather work boots into the plantar aspect of his foot. He reports pain on walking and notes swelling, warmth, and erythema on the top of his foot above the site of injury. He reports no fever or chills and has noted no drainage from the area of injury. Medical history is significant only for a fracture of the fourth and fifth metatarsal bones of the right foot 27 years ago treated with an internal fixation plate. His only medication is as-needed acetaminophen.
On physical examination, temperature is 38.1 °C (100.6 °F), blood pressure is 146/76 mm Hg, pulse rate is 89/min, and respiration rate is 16/min. BMI is 31. Examination is unremarkable except for the right foot, which is edematous, warm, and erythematous on the dorsal surface with a closed puncture wound on the plantar aspect of the foot directly beneath the first metatarsal-phalangeal area. The foot is tender to palpation but displays no drainage or fluctuance. The proximal leg is unremarkable, and no lymphadenopathy is detected.
Laboratory studies reveal a leukocyte count of 12,300/µL (12.3 × 109/L) and serum creatinine level of 1.0 mg/dL (88.4 µmol/L).
Plain radiograph of the right foot shows diffuse soft tissue swelling and metallic fixation plate. No gas is detectable in the soft tissues.
Which of the following is the most appropriate diagnostic test to perform next?