Renal papillary necrosis

A 50-year-old man comes to his primary care physician’s office for routine follow-up. He was recently diagnosed with osteoarthritis of the knees four weeks ago and was initiated on analgesic medications. Past medical history is significant for diabetes mellitus. Current medications include metformin, naproxen and acetaminophen. His vitals and physical exam are within normal limits. Laboratory results are as follows.  

Laboratory value	Results
	4 weeks ago	Today
Serum chemistry
Sodium	138 mEq/L	137 mEq/L
Potassium	4 mEq/L	4.1 mEq/L
Bicarbonate	24 mEq/L	25 mEq/L
Chloride	96 mEq/L	94 mEq/L
Creatinine	1.1 mg/dL	1.6 mg/dL
Urinalysis
Protein	100 mg/day	102 mg/day
Cast	None	None
Blood	negative	negative

   
Changes to which of the following anatomic regions is most likely accountable for this patient’s rise in creatinine?