A 35-year-old man presents with acute flank pain and hematuria. Non-contrast CT shows a 6 mm radiolucent stone in the right ureter at the pelvic brim. Which single feature best distinguishes calcium oxalate stones from uric acid stones?

Explanation

## Distinguishing Calcium Oxalate from Uric Acid Stones ### Key Discriminator: Radiopacity on Plain X-Ray **High-Yield:** The single best feature that distinguishes calcium oxalate stones from uric acid stones is **radiopacity on plain X-ray (KUB)**. Calcium oxalate stones are radiopaque (visible on KUB) due to their calcium content, whereas uric acid stones are radiolucent (invisible on KUB). This is the classic, textbook-standard distinguishing feature taught in every surgical and medical curriculum (Bailey & Love, Campbell-Walsh Urology). ### Why Option A is the Best Distinguisher Option A (radiopaque appearance on plain X-ray) is the **single most reliable and universally tested** feature that separates these two stone types. The clinical vignette itself highlights a "radiolucent stone" — pointing toward uric acid — precisely because calcium oxalate stones would have been visible on plain film. ### Comparative Table | Feature | Calcium Oxalate | Uric Acid | | --- | --- | --- | | **Radiopacity (KUB)** | **Radiopaque (visible)** | Radiolucent (invisible) | | **Solubility at alkaline pH** | Insoluble | Soluble (pH > 6.5) | | **Solubility at acidic pH** | Insoluble | Insoluble | | **Associated metabolic disorder** | Hyperoxaluria, hyperparathyroidism, hypercalciuria | Hyperuricemia, gout, low urine pH | | **Staghorn tendency** | Yes (common) | Less common | | **Treatment approach** | Hydration, dietary modification, thiazides | Alkalinization (sodium bicarbonate, allopurinol) | ### Why Option B is Incorrect as the "Best" Distinguisher While it is true that uric acid stones dissolve in alkaline urine (pH > 6.5) and calcium oxalate stones do not, **pH-dependent solubility is a property of uric acid stones alone** — it does not directly "distinguish" calcium oxalate from uric acid in the same binary, visually confirmable way that radiopacity does. Radiopacity is the primary, first-line distinguishing feature on imaging and is the standard answer in surgical textbooks (Bailey & Love's Short Practice of Surgery; Campbell-Walsh Urology). ### Why Other Options Are Incorrect - **Option C (Hyperparathyroidism):** Associated with calcium oxalate/calcium phosphate stones, but not a distinguishing feature between the two stone types in question. - **Option D (Staghorn calculi):** More classically associated with struvite (infection) stones; calcium oxalate can form staghorn calculi but this is not the best distinguishing feature vs. uric acid. **Clinical Pearl:** In the NEET PG/INI-CET context, "radiolucent stone" = uric acid stone; "radiopaque stone" = calcium oxalate stone. This is the single most tested distinguishing feature between these two stone types. **Mnemonic:** **CAOX-RAD** — Calcium Oxalate is RADiopaque; **URIC-LUCENT** — URIc acid is rLUCENT (radiolucent).