In a situation where a 3-month-old male has presented with lethargy and increased work of breathing, what is the next critical diagnostic step?

In the case of a 3-month-old male presenting with lethargy and increased work of breathing, it is crucial to understand the underlying potential causes of these symptoms. Lethargy and respiratory distress in an infant can arise from various conditions, including infections, metabolic disorders, and more serious concerns like congenital issues or trauma.

A skeletal survey is an important diagnostic tool when there is a concern for non-accidental injury (child abuse) or conditions like osteogenesis imperfecta that can present with lethargy and respiratory distress. In infants, the subtle signs of such conditions may not be readily apparent, and a skeletal survey helps to identify any unexplained fractures or signs of previous injuries that might account for the symptoms.

Other diagnostic steps such as a CT scan, urine analysis, or complete blood count may also provide valuable information but may not be as critical in this scenario. For instance, a CT scan would expose the infant to radiation and may not be the first-line investigation, while urine analysis may not address the immediate respiratory concerns. A complete blood count could help detect infections or anemia but does not specifically target the high suspicion of trauma or underlying skeletal issues that could be contributing to the presentation.

In summary, given the specific context of letharg