For a child with a suspected infection, which of the following is a key study to perform early on?

The most critical study to perform early on for a child with a suspected infection is a blood culture. This test is essential because it helps identify the presence of bacteria or other pathogens in the bloodstream, which can indicate a serious systemic infection like sepsis. Early identification and appropriate treatment of such infections are vital for improving outcomes in pediatric patients.

Collecting blood cultures is especially important because it allows for the detection of a wide range of potential pathogens, guiding antibiotic therapy. The earlier this information is obtained, the sooner targeted treatment can be initiated, which is crucial, especially in children, who can deteriorate rapidly when facing severe infections.

Other tests, such as a chest X-ray, may be useful if respiratory involvement is suspected, while an electrolyte panel can provide information about the child's metabolic status, and urinalysis can help identify urinary tract infections. However, none of these studies offer the broad initial overview of potential infections that blood cultures provide, making them secondary to the immediate need to assess for any life-threatening bacteremia or sepsis.