Q. A 45-year-old male presents to your clinic with a two-day history of left flank pain. The pain is described as a sharp, stabbing sensation that radiates to the groin. He denies any history of fevers, nausea, or vomiting. The pain is constant and has a severity of 8/10. He has a history of kidney stones and has had several episodes in the past. He admits to consuming a high-protein diet, which he believes may have contributed to his current episode. The patient also has a history of hypertension, which is well-controlled with lisinopril, and a history of gout for which he takes allopurinol. On physical examination, his temperature is 37 degrees Celsius (98.6 degrees Fahrenheit), blood pressure is 143/90 mm Hg, pulse 72/min, respiratory rate 18/min, T 98.6°F, O2 sat 97% on room air. The patient is alert and oriented, and in no acute distress. His cardiopulmonary examination is normal. Abdominal assessment reveals tenderness to palpation in the left flank with positive costovertebral angle tenderness. Normal bowel sounds are heard, and no masses are detected. There are no rashes or lesions noted, and his remaining examination is unremarkable. A urinalysis shows a moderate amount of blood, and is otherwise normal. You order blood work and an abdominal x-ray. Laboratory Results: Serum creatinine: 1.1 mg/dL Blood urea nitrogen: 25 mg/dL Serum uric acid: 8.5 mg/dL Abdominal X-ray Report: “Radiograph of abdomen in anteroposterior supine view taken. Normal bowel gas pattern. No evidence of dilatation. Vertebral column and pelvis appear normal. No evidence of nephrolithiasis. No cause for pain identified.” Which of the following is the most likely underlying etiology of this patient’s diagnosis?