A 26-year-old presents with systemic symptoms including night sweats and cough. What is the first diagnostic study to perform?

In the case of a 26-year-old presenting with systemic symptoms such as night sweats and cough, the most appropriate initial diagnostic study to perform is a chest X-ray. This imaging study is crucial for evaluating potential pulmonary causes of the patient's symptoms, particularly conditions like pneumonia, tuberculosis, or malignancies that could be contributing to the cough and night sweats.

A chest X-ray provides a quick, non-invasive way to assess for abnormalities in the lungs and can help identify areas of infection, masses, or other pathological changes that may require further investigation. Following the X-ray, if there are concerning findings, additional tests such as high-resolution CT scans or bronchoscopy could be warranted for a more detailed evaluation.

Other options, such as pulmonary function tests, would not be as immediate or relevant given the presence of acute systemic symptoms and are typically used to assess chronic respiratory conditions like asthma or COPD rather than acute infections. Bronchoscopy is an invasive procedure usually reserved for cases where there is a need for direct visualization or sampling of the airway and is not the first-line study for this clinical scenario. Hemoglobin and hematocrit tests focus on overall blood health and would not provide the necessary information regarding lung pathology in the context of the presenting symptoms.