A 52-year-old woman with a prosthetic aortic valve (mechanical, inserted 2 years ago) presents with a 4-week history of low-grade fever, night sweats, and progressive dyspnea. On examination, she has a new diastolic murmur and petechiae on her lower extremities. Blood cultures (×3 sets before antibiotics) are negative after 48 hours of incubation. Transthoracic echocardiography shows no obvious vegetation. What is the most appropriate next diagnostic step?