A 3-mo-old female infant has a history of poor feeding, shortness of breath during feedings, failure to thrive, and chronic cough. Physical examination reveals tachycardia and a gallop rhythm but no murmur. There is hepatomegaly but no cyanosis. A chest radiograph reveals cardiomegaly. The most appropriate diagnostic test is: A: Echocardiogram B: Electrocardiogram C: Blood culture D: Serum amino acids E: Urine pH