You are answering a USMLE-style medical question. Question: An 11-year-old boy was brought in by his mother with red tender bumps on his legs. The patient’s mother says that his symptoms started 3 days ago with a low-grade fever, malaise, and joint pain. He began to improve over the next 2 days, but this morning, when he woke up, she noticed multiple painful red bumps on his shins. Past medical history is significant for a recent severe sore throat and fever 1 week ago which resolved without treatment. The vital signs include: temperature 38.0°C (100.4°F), blood pressure 120/70 mm Hg, pulse 85/min, and respiratory rate 15/min. Physical examination reveals multiple, firm and tender erythematous nodules with indistinct borders, averaging 4 cm in diameter. Lesions are localized to the anterior surface of the lower legs. No evidence of drainage, bleeding, abscess formation or ulceration. Which of the following is the next best diagnostic step in this patient? Options: A) Throat culture B) Chest radiograph C) Bone marrow biopsy D) Colonoscopy Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.