USMLE® Step 1 Question of the Day: Histologic analysis

Prepare for the USMLE Step 1 exam with this detailed question about a 57-year-old woman with malaise and intermittent fevers over the past several weeks. Can you correctly identify which of the following findings is most likely to be demonstrated on histologic analysis?

A 57-year-old woman presents to her primary care physician with malaise and intermittent fevers over the past several weeks. She reports she was recently treated for a urinary tract infection, but her symptoms never completely improved. Her past medical history is significant for recurrent nephrolithiasis and type 2 diabetes mellitus. Her temperature is 38°C (100.4°F), pulse is 82/min, respirations are 16/min, blood pressure is 157/64 mmHg, and oxygen saturation is 99% on room air. Physical examination shows left-sided costovertebral angle tenderness on palpation. If a biopsy of the kidney was to be performed, which of the following findings is most likely to be demonstrated on histologic analysis?

Which of the following structures at the neuromuscular junction is most likely affected in this patient’s condition? 

A. Eosinophilic casts

B. Granulomatous tissue containing foamy macrophages

C. Neutrophilic infiltrate of the renal interstitium

D. Polygonal clear cells filled with lipids and carbohydrate

E. Mesangial proliferation

Scroll down for the correct answer!

The correct answer to today's USMLE® Step 1 Question is...

A. Eosinophilic casts  

Before we get to the Main Explanation, let's look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

B. Granulomatous tissue containing foamy macrophages 

Incorrect: These histologic findings are more consistent with xanthogranulomatous pyelonephritis, which is a rare form of chronic pyelonephritis commonly due to Proteus mirabilis infection. This disease is much rarer than generalized chronic pyelonephritis. 

C. Neutrophilic infiltrate of the renal interstitium  

Incorrect: These histologic findings are more consistent with acute pyelonephritis, which tends to present with acute onset nausea, vomiting, dysuria, and costovertebral angle tenderness. 

D. Polygonal clear cells filled with lipids and carbohydrate 

Incorrect: These are the histologic findings associated with renal cell carcinoma, which typically manifests with hematuria, weight loss, and polycythemia. 

E. Mesangial proliferation

Incorrect: These histologic findings are more consistent with IgA nephropathy, which is characterized by episodic hematuria that occurs concurrently with GI infection or upper respiratory infection due to the deposition of IgA in the mesangium.

Main Explanation

This patient presents with chronic fevers, fatigue, and costovertebral angle tenderness in the setting of treatment failure for a urinary tract infection, consistent with chronic pyelonephritis. Her history is also notable for recurrent nephrolithiasis, which is a known risk factor for this condition.   

Chronic pyelonephritis refers to the worsening kidney function and scarring that occurs in the setting of either recurrent urinary tract infections (most commonly), poorly treated urinary tract infections, an impaired vesicoureteral reflux mechanism, or chronically obstructing kidney stones. Gross specimens will demonstrate coarse, asymmetric corticomedullary scarring and blunted calyces. Renal tubules often contain eosinophilic casts which resemble thyroid tissue, coined thyroidization of the kidney.

Major Takeaway

Chronic pyelonephritis refers to worsening kidney function and scarring that occurs most commonly due to recurrent urinary tract infections. The major histologic finding is eosinophilic casts termed thyroidization of the kidney.  

References

• Curry, M. (2017, May 18). Rosen's Emergency Medicine: Concepts and Clinical Practice. Retrieved from https://www.us.elsevierhealth.com/rosens-emergency-medicine-concepts-and-clinical-practice-9780323354790.html. ISBN: 0323354793 
• Urinary Tract Infection Syndromes Betsy Foxman PhD Infectious Disease Clinics of North America, 2014-03-01, Volume 28, Issue 1, Pages 1-13, doi: 10.1016/j.idc.2013.09.003. 
• International clinical practice guidelines for the treatment of acute uncomplicated cystitis and pyelonephritis in women: A 2010 update by the Infectious Diseases Society of America and the European Society for Microbiology and Infectious Diseases. Gupta K1, Hooton TM, Naber KG, Wullt B, Colgan R, Miller LG, Moran GJ, Nicolle LE, Raz R, Schaeffer AJ, Soper DE; Infectious Diseases Society of America; European Society for Microbiology and Infectious Diseases.Clin Infect Dis. 2011 Mar 1;52(5):e103-20. doi: 10.1093/cid/ciq257

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