Renal papillary necrosis

A 50-year-old man comes to his primary care PA’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	Reference range
	4 weeks ago	Today
Serum chemistry
Sodium	138 mEq/L	137 mEq/L	136-146 mEq/L
Potassium	4 mEq/L	4.1 mEq/L	3.5-5 mEq/L
Bicarbonate	24 mEq/L	25 mEq/L	22-28 mEq/L
Chloride	96 mEq/L	94 mEq/L	95-105 mEq/L
Creatinine	1.1 mg/dL	1.6 mg/dL	0.6-1.2 mg/dL
Urinalysis
Protein	100 mg/day	102 mg/day	<150 mg/day
Cast	None	None	None
Blood	None	None	None

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