A 5-month-old boy has episodes of turning blue during crying. What is the first-line diagnostic test for his condition?

In infants who experience episodes of turning blue during crying, this is often indicative of a condition known as cyanotic spells or cyanosis. The most common underlying causes for this symptom in an infant include congenital heart defects, such as tetralogy of Fallot or other structural abnormalities affecting oxygenation.

Echocardiography is the first-line diagnostic test in this scenario because it allows for a comprehensive assessment of the heart's structure and function. This imaging test can visualize the heart chambers, valves, and great vessels, providing critical information about any potential congenital heart defects or anomalies that may explain the blue spells. It can also evaluate blood flow patterns and the presence of shunting, which is crucial in understanding the etiology of cyanosis.

While other tests like electrocardiography and chest radiography can provide some information about heart rhythm and cardiac silhouette, respectively, they do not offer the detailed visualization of cardiac structures necessary to diagnose congenital heart conditions. Cardiac catheterization is more invasive and typically reserved for cases where non-invasive modalities do not provide conclusive results or for therapeutic interventions.