In a 3-month-old with a harsh pansystolic murmur, which initial study is best to diagnose a potential ventricular septal defect?

In the assessment of a suspected ventricular septal defect (VSD) in a 3-month-old infant, an echocardiogram is the most appropriate initial test. This non-invasive imaging modality allows for real-time visualization of the heart's structure and function. The echocardiogram provides critical information about the presence of a ventricular septal defect, including its size, location, and potential impact on cardiac function. This is particularly important in an infant, as the early detection and assessment of any cardiac abnormalities can guide immediate management and intervention if necessary.

Other diagnostic tests like a CT angiogram or cardiac catheterization are typically more invasive and not first-line choices for initial diagnosis in infants. An electrocardiogram, while useful for assessing heart rhythm and electrical activity, does not provide the structural information necessary to confirm a VSD. Therefore, the echocardiogram stands out as the optimal option for both its effectiveness in diagnosing the defect and its safety for the patient.