Ease Into Prep with a USMLE^® Step 2 CK Question of the Day

The correct answer is D. This patient provides a classic history for someone at risk for struvite kidney stones. Struvite stones are a result of urinary infection with bacteria, usually Proteus species, which possess urease, an enzyme that degrades urea to NH₃ and CO₂. The NH₃ hydrolyzes to NH₄⁺ and increases urine pH to 8 or 9. The CO₂ hydrates to H₂CO² and then dissociates to CO₃ ^2-, which precipitates with calcium as CaCO₃. The NH₄⁺ precipitates PO₄^3- and Mg2+ to form MgNH₄ PO₄ (struvite). The result is a stone of calcium carbonate admixed with struvite. Struvite does not form in urine in the absence of infection, because NH₄⁺ concentration is low in urine that is alkaline in response to physiologic stimuli. These stones are common in immobilized patients such as this woman with multiple sclerosis. In such cases, the stones can be bilateral.

Chronic Proteus infection can occur because of impaired urinary drainage, urologic instrumentation or surgery, and especially with chronic antibiotic treatment, which can favor the dominance of Proteus in the urinary tract. The formation of these struvite stones in the renal collecting system leads to staghorn configuration typically seen with this type of stone. These stones are also called infective stones, because they contain numerous infective bacteria within their structure where antibiotics cannot penetrate. Because the antibiotics cannot penetrate the stone, the stones must be removed if the infection is to be cured. Prophylaxis against recurring struvite stones requires the maintenance of sterile urine, high urine volumes, and decreased urinary phosphate levels.

Calcium oxalate, as either monohydrate or dihydrate (less dense), is a major component of most urinary stones (choices A and B). Although these metabolites comprise a majority of all stones formed, they are not normally seen in patients who have recurrent urinary tract infections and staghorn calculi.

Cystine stones (choice C) form in patients who have cystinuria, which is the result of an inherited autosomal recessive defect in the renal tubular reabsorption of four amino acids: cystine, ornithine, lysine, and arginine (mnemonic: COLA). Patients who are homozygous for the disease excrete increased levels of cystine. The stones form in acid urine.

Uric acid stones (choice E) form in patients who have high urinary uric acid levels. This may occur in those who have gout, myeloproliferative disorders (leukemia and lymphoma), those on chemotherapy, or patients who have chronic diarrhea. These stones form in acidic urine (pH <5.5) and are radiolucent (i.e., they are not seen on regular plain radiographs). The patient in this scenario had stones visualized on radiograph.