Final answer:
The most likely cause of the infant's respiratory distress with symptoms of fever, cough, stridor, and clear lungs could be croup or a viral infection such as RSV. Pneumonia is less likely given the auscultation findings.
Step-by-step explanation:
An infant presenting with a fever, cough, increased work of breathing, alertness with stridor and retractions, and clear lungs to auscultation most likely has a form of respiratory distress. Given that there is no mention of prematurity and based on the symptoms described, this could indicate a few possible conditions such as croup, a viral respiratory infection characterized by stridor, a barking cough, and retractions that represent increased work of breathing.
However, conditions like bronchiolitis, especially caused by the Respiratory Syncytial Virus (RSV), can also present with similar symptoms in infants. It is less likely to be pneumonia due to the lungs being clear upon auscultation, although atypical or walking pneumonia can sometimes present without obvious signs of consolidation on examination. A definitive diagnosis would typically involve a thorough clinical examination and possibly imaging studies or viral testing, depending on the clinical judgment of the healthcare provider.