A 6-week-old infant is presented with poor feeding and fever. What is the likely diagnostic test anticipated?

In the case of a 6-week-old infant presenting with poor feeding and fever, the primary concern is the possibility of a serious infection, such as meningitis. Given the age of the infant and the symptoms, a lumbar puncture is likely anticipated as it allows for the collection of cerebrospinal fluid (CSF). Analyzing the CSF can confirm or rule out conditions like meningitis, which is critical given the age of the patient and the potentially severe implications of this diagnosis.

A lumbar puncture can help determine the presence of pathogens, white blood cell count, and protein levels in the CSF, providing essential diagnostic information. This procedure is particularly important in young infants, where symptoms can be nonspecific and prompt action is necessary to ensure appropriate treatment.

While other tests such as blood cultures can indeed provide valuable information about systemic infections and blood-borne pathogens, they do not offer the same direct insight into central nervous system infections as a lumbar puncture does. Therefore, in this scenario, the lumbar puncture is the most critical and anticipated diagnostic test to accurately assess the infant's condition.