Q. A 45-year-old male presented to the emergency department 4 days ago with severe left flank pain, nausea, and vomiting. He reported the pain was present for 48 hours and was increasing in intensity. He also reported he was passing small amounts of blood in his urine. The pateint was previously healthy and denied taking any medications regularly. A urinalysis showed the presence of white blood cells and red blood cells. He was diagnosed with a urinary tract infection and prescribed levofloxacin. The patient returns to the emergency department today with complaints of more severe left flank pain, and persistent nausea and vomiting. On physical examination, he is ill-appearing and weak. His temperature is 38.2 degrees Celcius (100.8 degrees Farenheit), blood pressure is 90/50 mm Hg, respiratory rate 25/min, heart rate 100/min, and oxygen saturation is 98 percent on room air. The patients lungs are clear to auscultation bilaterally, and his cardiac assessment is unremarkable. His abdomen is soft, mildly obese, and tender to palpation in the left upper quadrant with moderate rebound tenderness appreciated. You also note moderate left costovertebral angle tenderness, and detect a mass on his left flank. Based on the information you have at this point in time, which of the following is the most likely underlying etiology of this patient’s symptoms?