Nephrolithiasis questions love to bait you with urine pH, crystal shape, and a patient vignette (gout, UTI, malabsorption, genetic kid). The fastest way to stay oriented is to have a “stone map” in your head—then plug in the details.
The 10-second acronym: C.U.S.C. = “CUSC stones”
Think: Calcium oxalate, Uric acid, Struvite, Cystine.
Visual mnemonic: the “Stone Shelf”
Picture a shelf with four labeled jars:
- CaOx jar: envelope crystals + a small dumbbell (two common shapes)
- Uric acid jar: rhomboids/needles + a gouty toe
- Struvite jar: a coffin lid + a bacteria icon
- Cystine jar: hexagons + a child running (genetic)
If you can recall shape + trigger + pH, you’ll rarely miss these.
Quick-hit one-liners (what to say in your head on test day)
- Calcium oxalate: “Most common; envelopes/dumbbells; forms in normal to acidic urine—think ethylene glycol, vitamin C, malabsorption.”
- Uric acid: “Rhomboids/needles in acidic urine; radiolucent; think gout, high cell turnover, tumor lysis.”
- Struvite: “Coffin lids in alkaline urine from urease+ bugs; can form staghorn calculi.”
- Cystine: “Hexagons from COLA amino acid reabsorption defect; acidic urine; cyanide nitroprusside positive.”
The high-yield table (USMLE-friendly)
| Stone type | Urine pH tendency | Crystal shape | Radiopacity | Classic associations | Big buzzwords |
|---|---|---|---|---|---|
| Calcium oxalate | Normal → acidic | Envelope or dumbbell | Radiopaque | Ethylene glycol, vitamin C excess, Crohn/malabsorption, bariatric surgery | Most common stone |
| Uric acid | Acidic | Rhomboid or needles | Radiolucent | Gout, high purine diet, myeloproliferative states, tumor lysis | “Radiolucent + acidic” combo |
| Struvite (MgNH₄PO₄) | Alkaline | Coffin lid | Radiopaque | UTI with urease+ (Proteus, Klebsiella, Staph saprophyticus) | Staghorn calculi |
| Cystine | Acidic | Hexagon | Faintly radiodense (can be less obvious) | Cystinuria (PCT transport defect) | COLA, nitroprusside test |
Urine pH hacks (fast pattern recognition)
If the question gives you a urine pH, use it as a filter:
- Alkaline urine → think Struvite (especially with UTI symptoms).
- Acidic urine → think Uric acid or Cystine
- Gout/high cell turnover = uric acid
- Child/teen + recurrent stones = cystine
Calcium oxalate can occur across pH ranges, so when you see envelope crystals or classic risk factors, lock it in.
Pharmacology tie-ins (Step 1/2 favorite angles)
Prevention & treatment pearls
- Thiazides ↓ urinary calcium → help prevent calcium stones (esp. hypercalciuria).
- Potassium citrate alkalinizes urine → helps prevent/treat:
- Uric acid stones (they dissolve with alkalinization)
- Cystine stones (more soluble at higher pH)
- Allopurinol (xanthine oxidase inhibitor) lowers uric acid → prevention for recurrent uric acid stones or hyperuricosuria.
- Acetohydroxamic acid inhibits urease (rarely used) → can help with struvite stones when infection can’t be eradicated easily (think complicated/recurrent).
“Radiolucent” trap you should know
- Uric acid stones are radiolucent on plain X-ray but visible on non-contrast CT.
- Calcium and struvite are typically radiopaque.
Micro tie-in: urease bugs and struvite
Urease splits urea → ammonia, which alkalinizes urine and promotes MgNH₄PO₄ precipitation.
High-yield urease organisms:
- Proteus (classic)
- Klebsiella
- Staph saprophyticus
Clinical picture: recurrent UTIs + alkaline urine + staghorn calculus risk.
Genetics tie-in: cystinuria in one breath
Cystinuria = defective PCT reabsorption of COLA amino acids:
- Cystine
- Ornithine
- Lysine
- Arginine
Only cystine is poorly soluble → hexagon crystals and recurrent stones, often starting young.
Rapid-fire self-test (30 seconds)
- Coffin-lid crystals + alkaline urine + recurrent UTI → Struvite
- Radiolucent stone + gout + acidic urine → Uric acid
- Hexagon crystals in a teen → Cystine
- Envelope crystals + Crohn disease/bariatric surgery → Calcium oxalate