A 42-year-old man with a history of Crohn disease and multiple prior ileal resections pres… — USMLE Step 2 CK

A $42$ -year-old man with a history of Crohn disease and multiple prior ileal resections presents to the clinic for follow-up of a recent episode of right-sided flank pain. The pain began two days ago, radiated to his groin, and has since resolved completely after he took ibuprofen. He has chronic, non-bloody diarrhea but currently feels well. His temperature is $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), blood pressure is $120/80 \text{ mm Hg}$ , and pulse is $78/\text{min}$ . Physical examination demonstrates a soft, non-tender abdomen with mild right costovertebral angle tenderness. Laboratory testing reveals a serum creatinine of $0.9 \text{ mg/dL}$ . Urinalysis shows a pH of $5.0$ , $15-20 \text{ RBCs/hpf}$ , and no leukocyte esterase or nitrites. A kidney, ureter, and bladder (KUB) radiograph demonstrates no radiopaque abnormalities. A subsequent non-contrast CT scan of the abdomen and pelvis reveals a $1.6\text{-cm}$ calculus in the right renal pelvis causing mild hydronephrosis. Which of the following is the most appropriate next step in management of this patient's calculus?

Oral potassium citrateAnswer
B
Extracorporeal shock wave lithotripsy
C
Percutaneous nephrolithotomy
D
Ureteroscopic stone extraction
E
Oral allopurinol

Answer

Oral potassium citrate

The correct answer is oral potassium citrate because the patient's presentation is classic for a uric acid stone, which can be managed medically rather than surgically. Chronic diarrhea from Crohn disease leads to significant gastrointestinal bicarbonate loss. This causes a chronic metabolic acidosis, resulting in highly acidic urine (pH 5.0) as the kidneys attempt to compensate. Uric acid is highly insoluble in acidic environments and readily precipitates. Characteristically, uric acid stones are radiolucent (invisible on KUB radiographs) but can be clearly visualized on CT scans. While a 1.6-cm renal calculus of any other composition would typically require surgical intervention (such as ESWL or PCNL), large uric acid stones in stable, uninfected patients are the major exception. They can be completely dissolved through urinary alkalinization (raising the pH to 6.0-6.5) using oral potassium citrate, effectively treating the stone without invasive procedures.

Step-by-Step Solution

Analyze the patient's history and imaging findings to identify the stone composition.

The patient has Crohn disease with chronic diarrhea (leading to metabolic acidosis and acidic urine), a radiolucent stone on KUB, and a visible stone on CT.

This specific triad of findings is highly diagnostic for a uric acid stone, which behaves differently from calcium oxalate or struvite stones.

Evaluate the clinical status and stone size/location.

The patient is hemodynamically stable, currently pain-free, with normal renal function and a 1.6-cm non-infected renal pelvic stone.

This confirms there is no indication for emergent decompression (e.g., nephrostomy) and allows for elective definitive therapy.

Select the definitive therapy based on stone composition.

Urinary alkalinization with oral potassium citrate.

Uric acid solubility increases exponentially as urine pH rises above 6.0. Alkalinization can completely dissolve large uric acid stones, sparing the patient from surgery.

Key Concept

Diagnosis and non-surgical management of uric acid nephrolithiasis via urinary alkalinization.

Estimated Time:2m 0s

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