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PATIENT 
Mr. K is an 80-year-old man brought in by his family with the chief complaint of worsening malaise, anorexia, and confusion for the past 3 days. He is generally healthy and independent, but over the past 2 months has had decreased energy and vague abdominal pain. Over the last 3 days, he has been complaining of pain in his right flank, for which he has been taking ibuprofen. He has been drinking liquids but not eating much. His past medical history is notable for long-standing hypertension, BPH, and remote colorectal cancer. He takes losartan, amlodipine, and finasteride. On physical exam, he is alert but confused. His BP is 160/80 mm Hg, pulse is 88 bpm, RR is 16 breaths per minute, and he is afebrile. There is no adenopathy, lungs are clear, and cardiac exam is normal. Abdominal exam shows no masses or tenderness; there is mild right-sided abdominal tenderness with normal bowel sounds. His prostate is mildly enlarged, without nodules. There is no peripheral edema.
Initial laboratory test results include Na, 138 mEq/L; K, 4.8 mEq/L; Cl, 100 mEq/L; HCO3, 20 mEq/L; BUN, 90 mg/dL; creatinine, 7.2 mg/dL, up from his baseline of 1.5 mg/dL.
At this point, what is the leading hypothesis, what are the active alternatives, and is there a must not miss diagnosis? Given this differential diagnosis, what tests should be ordered?
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RANKING THE DIFFERENTIAL DIAGNOSIS
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All 3 etiologies of AKI need to be considered. His age, prostatic enlargement, and vague complaints of abdominal and flank pain are all pivotal points suggesting urinary tract obstruction. However, he also could have prerenal AKI from either NSAID use or intravascular volume depletion. Furthermore, renal hemodynamics are altered by angiotensin receptor blockers which can contribute to prerenal AKI. He has no history suggesting a specific intrarenal cause, although the history of hypertension puts him at increased risk. Thus, intrarenal causes would be considered only if no postrenal or prerenal cause could be identified, or if the urinalysis were suggestive (granular or cellular casts). Table 28-6 lists the differential diagnosis.
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Antihypertensive agents are discontinued. Mr. K’s urine sodium is 20 mEq/L, with a FENa of 1%. He is given ...