Final answer:
To assess the severity of pulmonary agenesis in a newborn, the RT should recommend a chest X-ray, which is a suitable diagnostic method to visualize the lungs' structure and any malformations.
Step-by-step explanation:
The diagnosis of pulmonary agenesis in a newborn would typically warrant the use of diagnostic tests to assess the severity of the malformation. Among the options provided, the RT (Respiratory Therapist) should recommend a chest X-ray, which is a non-invasive imaging test that can reveal the structure of the lungs and help determine the presence and extent of the agenesis. Pulmonary function tests are typically not feasible in newborns, especially those in the NICU. An arterial blood gas analysis can provide information on the baby's oxygenation and ventilation status but would not directly assess the malformation. A ventilation-perfusion scan could assess blood flow and air distribution within the lungs but could be challenging to perform in a critically ill newborn. Therefore, out of the given options, a chest X-ray is the most appropriate diagnostic test to recommend.