A 6-month-old presents with a fever and significant irritability without upper respiratory symptoms. What is the most likely diagnosis?

In this scenario, a 6-month-old infant presenting with fever and significant irritability, without upper respiratory symptoms, suggests a potential diagnosis of a urinary tract infection (UTI). UTIs are common in infants and can manifest primarily with fever and irritability rather than the classic urinary symptoms that are more easily identified in older children and adults.

In this age group, UTIs can occur due to various reasons, including anatomical predispositions or lack of toilet training, making them less likely to exhibit symptoms such as dysuria or increased urinary frequency. The significant irritability is particularly telling, as infants with UTIs may not only be feverish but also uncomfortable and fussy, indicating a possible underlying infection.

Other conditions like dehydration, bronchiolitis, and gastroenteritis may present with distinct additional symptoms or signs that would help narrow down the diagnosis. For instance, bronchiolitis typically presents with respiratory distress or wheezing, gastroenteritis would likely involve vomiting and diarrhea, and dehydration would often be accompanied by signs of fluid loss such as dry mucous membranes or decreased urination. The absence of these additional symptoms strongly points towards a UTI as the most likely diagnosis in this case.