<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 62-year-old man is brought to the emergency department with fatigue, dry cough, and shortness of breath for 3 days. He reports a slight fever and has also had 3 episodes of watery diarrhea earlier that morning. Last week, he attended a business meeting at a hotel and notes some of his coworkers have also become sick. He has a history of hypertension and hyperlipidemia. He takes atorvastatin, hydrochlorothiazide, and lisinopril. He appears in mild distress. His temperature is 102.1F 38.9C, pulse is 56min, respirations are 16min, and blood pressure is 15085 mm Hg. Diffuse crackles are heard in the thorax. Examination shows a soft and nontender abdomen. Laboratory studies show:Hemoglobin 13.5 gdLLeukocyte count 15,000mm3Platelet count 130,000mm3SerumNa 129 mEqLCl- 100 mEqLK 4.6 mEqLHCO3- 22 mEqLUrea nitrogen 14 mgdLCreatinine 1.3 mgdLAn x-ray of the chest shows infiltrates in both lungs. Which of the following is the most appropriate next step in diagnosis?? A: Direct immunofluorescent antibody test, B: Stool culture, C: Polymerase chain reaction, D: CT Chest, E: Urine antigen assay,  <response>: This is a clinical vignette describing a 62-year-old man with fever, cough, diarrhea and abnormal lung exam findings concerning for pneumonia. Key aspects of the history include recent travel, exposure to sick contacts, and vital sign abnormalities including fever and hypoxemia. The chest X-ray shows bilateral infiltrates concerning for pneumonia. The question asks for the most appropriate next step in diagnosis. Based on the clinical presentation, the main differential diagnosis includes infectious pneumonia versus non-infectious pneumonia. To narrow the differential and guide antibiotic therapy, identifying the infectious organism would be the next step. Choice A tests for atypical organisms but is not first-line. Choice B could identify a gastrointestinal pathogen causing the diarrhea but does not help diagnose the pneumonia. Choice C is reasonable but not widely available as a rapid test. Choice D provides imaging but does not identify the organism. Choice E is the best option as the urine antigen assay can rapidly diagnose Legionella pneumonia.E: Urine antigen assay 