A 38-year-old man from Delhi presents with acute flank pain and hematuria. Non-contrast CT shows a 6 mm stone in the mid-ureter with mild hydronephrosis. Serum calcium and phosphate are normal. Urine pH is 7.8 with positive nitrites and pyuria. Which finding best distinguishes struvite stones from calcium oxalate stones in this clinical context?

## Distinguishing Struvite from Calcium Oxalate Stones ### Clinical Context Analysis The patient presents with: - Alkaline urine (pH 7.8) - Positive nitrites and pyuria (signs of UTI) - Normal serum calcium and phosphate - Acute nephrolithiasis ### Key Discriminating Feature **Key Point:** Struvite stones (magnesium ammonium phosphate) form specifically in alkaline urine in the setting of urinary tract infection caused by urease-producing bacteria (Proteus, Klebsiella, Pseudomonas). This combination of **alkaline urine + UTI** is pathognomonic for struvite stone formation. ### Comparative Table | Feature | Struvite (Magnesium Ammonium Phosphate) | Calcium Oxalate | |---------|----------------------------------------|------------------| | **Urine pH** | Alkaline (pH > 7.0) | Acidic to neutral (pH 5.5–7.0) | | **UTI Association** | Strongly associated (urease-producing organisms) | Not associated with UTI | | **Bacteria** | Proteus, Klebsiella, Pseudomonas (urease+) | None | | **Nitrites in urine** | Positive (indicates UTI) | Negative | | **Pyuria** | Present | Absent (unless concurrent UTI) | | **Serum calcium** | Normal | May be elevated (hypercalciuria) | | **Radiographic appearance** | Radiopaque (staghorn calculi common) | Radiopaque | | **Clinical course** | Recurrent if UTI untreated | Metabolic recurrence | ### Pathophysiology **High-Yield:** Urease-producing bacteria cleave urea → ammonia + CO₂, raising urine pH and promoting precipitation of magnesium ammonium phosphate. This is why struvite stones are called **"infection stones"** — they cannot form without UTI. ### Clinical Pearl **Mnemonic:** **STRUVITE = Staghorn + Urease-producing bacteria + Infection** - **S**taghorn morphology (large, branching) - **T**riple phosphate (Mg²⁺ + NH₄⁺ + PO₄³⁻) - **U**rease-producing bacteria - **V**ery alkaline urine - **I**nfection (UTI) - **T**reatment: antibiotics + stone removal - **E**radication of UTI prevents recurrence ### Why This Matters **Tip:** On NEET PG, the presence of **alkaline urine + UTI (nitrites + pyuria)** in a stone-forming patient is a red flag for struvite stone. Calcium oxalate stones do NOT require UTI and form in normal to acidic urine. [cite:Campbell-Walsh Urology 12e Ch 52]