A 27-year-old woman presents with 10 days of painful, tender nodules on the anterior aspects of both shins. She reports low-grade fever, fatigue, and mild ankle arthralgias. She denies cough, dyspnea, hemoptysis, or recent travel. She is not taking any new medications. Physical examination shows several 2–4 cm erythematous, warm, exquisitely tender subcutaneous nodules on both shins without ulceration. No mucosal lesions are present. The clinician suspects erythema nodosum. Which of the following is the most appropriate initial imaging study to evaluate for an associated underlying condition?