Renal/ureteral calculi causing acute flank pain; calcium oxalate most common.
Also known as: urolithiasis, kidney stones, nephrolithiasis, ureterolithiasis, renal colic
Overview
Formation of crystalline aggregates (calculi) within the urinary tract — kidneys, ureters, bladder, or urethra. Acute presentations are dominated by ureteral stones causing renal colic, with calcium oxalate stones accounting for ~75% of all calculi.
Epidemiology
Lifetime prevalence ~10% in men, ~7% in women. Peak incidence ages 20-50. Recurrence ~50% within 10 years without prevention. Rising in women and adolescents over recent decades.
Try two board-style Urolithiasis questions
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Question 1RenalMedium
A 50-year-old man presents to the ED with sudden-onset severe left flank pain radiating to the groin, accompanied by nausea and vomiting. He reports gross hematuria. Vital signs are stable. Urinalysis shows microscopic and gross hematuria without pyuria. Non-contrast CT demonstrates a 6-mm stone in the left distal ureter with mild ipsilateral hydronephrosis and no evidence of infection. Which of the following is the most appropriate initial management?
AMedical expulsive therapy
BAggressive intravenous hydration
CPercutaneous nephrostomy
DImmediate ureteroscopy
EExtracorporeal shock wave lithotripsy
Reveal answer & full explanation
Correct answer: A — Medical expulsive therapy
AMedical expulsive therapy✓
BAggressive intravenous hydration
CPercutaneous nephrostomy
DImmediate ureteroscopy
EExtracorporeal shock wave lithotripsy
Why Medical expulsive therapy is correct
Ureteral stone 5–10 mm: only about 47–60% pass spontaneously
Medical expulsive therapy (MET): tamsulosin (alpha-1 blocker) relaxes ureteral smooth muscle, improves stone passage rate, and reduces analgesic need
AUA guideline: offer alpha-blocker MET for uncomplicated distal ureteral stones ≤10 mm; EAU 2026 strongly recommends it for distal stones 5–10 mm, where the benefit is concentrated; evidence of benefit is weaker for proximal ureteral stones
NSAIDs (nonsteroidal anti-inflammatory drugs) are superior to opioids for ureteral colic pain
Why the others are wrong
Extracorporeal shock wave lithotripsy — a definitive procedure for stones that fail to pass or are larger, not first-line for a small stone likely to pass with MET
Percutaneous nephrostomy — reserved for an obstructing stone with infection/sepsis or acute kidney injury, neither present here
Immediate ureteroscopy — invasive intervention reserved for failed conservative therapy or larger/refractory stones; with infection or sepsis, the obstructed system is decompressed first and definitive ureteroscopy is deferred
Aggressive intravenous hydration — flushing the stone out with large fluid volumes is an intuitive reflex, especially in a patient who is vomiting, but high-volume hydration or forced diuresis has not been shown to speed ureteral stone passage or relieve colic; fluids replace losses as needed, while an alpha-blocker is what improves passage of a 6-mm distal stone
Additional high-yield points
Indications for urgent urology referral: obstruction plus infection (fever), solitary kidney, renal failure, intractable pain/vomiting, or bilateral obstructing stones; a stone >10 mm or one that fails to pass with observation/MET warrants elective (not urgent) urologic intervention
Question 2RenalMedium
A 45-year-old man presents after his second calcium oxalate kidney stone. A 24-hour urine collection shows urinary citrate 120 mg/day (low; normal greater than 320 mg/day), normal calcium and oxalate, and urine pH 6.4. Stone analysis confirms calcium oxalate monohydrate. Which of the following is the most appropriate pharmacotherapy to prevent recurrence?
APotassium citrate
BSodium bicarbonate
CAcetazolamide
DAllopurinol
EHydrochlorothiazide
Reveal answer & full explanation
Correct answer: A — Potassium citrate
APotassium citrate✓
BSodium bicarbonate
CAcetazolamide
DAllopurinol
EHydrochlorothiazide
Why Potassium citrate is correct
The 24-hour urine shows isolated hypocitraturia (citrate 120 mg/day) with normal calcium and oxalate.
Citrate binds urinary calcium and raises urine pH, inhibiting calcium oxalate and calcium phosphate crystallization; potassium citrate is first-line for hypocitraturia per current urologic guidance.
Fluid intake above 2.5 L/day remains the cornerstone measure for all stone types.
Why the others are wrong
Sodium bicarbonate - an alkali that can raise urinary citrate, but its sodium load increases urinary calcium excretion, so it is reserved for patients who cannot tolerate a potassium salt (right-alkali, wrong-salt trap).
Allopurinol - used for hyperuricosuria or uric acid stones, neither of which is present here (buzzword trap on 'stones').
Hydrochlorothiazide - reduces urinary calcium and is first-line for hypercalciuria, but calcium excretion is normal here (premature closure on the most common stone therapy).
Acetazolamide - a carbonic anhydrase inhibitor that alkalinizes the urine, which can make it look like a substitute for citrate therapy, but the metabolic acidosis it causes lowers urinary citrate further and promotes calcium phosphate stones (alkalinization-without-citrate trap).
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Recurrent UTIs with urease-producing organisms (Proteus, Klebsiella) — struvite stones
Crohn disease, malabsorption, bariatric surgery (enteric hyperoxaluria)
Family history, obesity, diabetes, hot climate
Pathophysiology
Stone formation requires urinary supersaturation of stone-forming salts (calcium, oxalate, uric acid) plus inadequate inhibitors (citrate, magnesium). Calcium oxalate stones typically grow on Randall plaques (interstitial calcium phosphate deposits at the renal papillary tip) or form by crystal aggregation within tubular lumens. Growing stones obstruct the ureter, causing distention, prostaglandin release, smooth muscle spasm, and pain referred along splanchnic nerves (T11-L1).
Clinical presentation
Symptoms
Sudden severe colicky flank pain radiating to groin/testicle/labia
Patient writhing, unable to find comfortable position (contrast with peritonitis where patients lie still)
Nausea and vomiting
Dysuria, urinary frequency/urgency if stone in distal ureter near bladder
Gross hematuria in 30-40%, microscopic in ~85%
Signs / physical exam
CVA tenderness (often mild despite severe pain)
Patient appears restless and uncomfortable
Vital signs may show mild tachycardia, hypertension from pain
Fever raises concern for obstructed infected system (urologic emergency)
Classic findings
Writhing patient with acute severe flank pain radiating to groin + hematuria = textbook ureteral stone.
Ruptured abdominal aortic aneurysm — Older patients, hypotension, pulsatile mass; consider in older patients with first-ever 'stone' pain
Ovarian torsion / ectopic pregnancy — Women of reproductive age; pelvic exam, beta-hCG, transvaginal US
Appendicitis — Right-sided pain migrating to RLQ, fever, leukocytosis, may mimic right ureteral stone
Renal infarction — Risk factors for embolism (Afib, endocarditis); elevated LDH; CT with contrast diagnostic
Musculoskeletal pain — Reproducible with palpation, recent trauma or activity, normal UA
Diagnostic workup
Diagnostic criteria
CT-confirmed urinary tract calculus with consistent clinical presentation. Size and location predict spontaneous passage: <5 mm pass ~80%, 5-10 mm ~50%, >10 mm rarely; distal ureteral stones pass more often than proximal.
Labs
Urinalysis with microscopy — hematuria (gross or microscopic), pH (acidic favors uric acid; alkaline favors struvite, calcium phosphate), crystals
Urine culture if pyuria or fever
BMP — Ca, creatinine, electrolytes
CBC — leukocytosis suggests infection
Beta-hCG in women of reproductive age (before imaging)
Stone analysis when stone retrieved
Metabolic workup (24-h urine for Ca, oxalate, citrate, uric acid, Na, volume) for recurrent stones, single kidney, or first stone in child
Imaging
Non-contrast CT abdomen/pelvis (low-dose) — gold standard; identifies stones >1 mm, hydronephrosis, alternative diagnoses
Renal ultrasound — first-line in pregnancy and children; reasonable initial test in adults to detect hydronephrosis (lower sensitivity for small ureteral stones)
KUB plain film — limited; misses radiolucent uric acid stones
MR urography — alternative in pregnancy when US insufficient
Diagnostic algorithm
Stone Type
Prevalence
Urine pH
Crystals
Prevention
Calcium oxalate
~75%
Variable
Envelope/dumbbell-shaped
Hydration, thiazide, citrate, normal Ca diet
Calcium phosphate
~10%
Alkaline (>6.5)
Amorphous, wedge-shaped
Treat distal RTA, hyperparathyroidism
Uric acid
~10%
Acidic (<5.5)
Rhomboid/needle (radiolucent)
Alkalinize urine (K-citrate), allopurinol
Struvite (Mg-NH4-PO4)
~5-10%
Alkaline (>7)
Coffin-lid
Treat UTI, complete surgical removal
Cystine
<1%
Acidic
Hexagonal
Hydration, alkalinize, tiopronin/penicillamine
Kidney stone types with characteristic urine pH, crystal morphology, and prevention strategies.
Treatment
First-line
Pain control — NSAIDs (ketorolac IV/IM, ibuprofen) are first-line; superior to opioids and reduce ureteral spasm; AVOID in CKD, dehydration, or single kidney
Opioids (morphine, hydromorphone) as second-line or adjunctive analgesia
Antiemetics (ondansetron, metoclopramide)
IV fluids only if dehydrated — aggressive forced diuresis does not promote passage
Medical expulsive therapy (MET): alpha-blocker — tamsulosin 0.4 mg daily — for uncomplicated distal ureteral stones ≤10 mm (greatest benefit for 5-10 mm stones; weaker evidence for proximal stones), alongside observation with NSAIDs and follow-up imaging
Second-line / adjunct
Obstructed + infected system (fever, sepsis, or other signs of infection) = EMERGENT decompression (ureteral stent or percutaneous nephrostomy) plus IV antibiotics and resuscitation; delay definitive stone treatment until the infection clears. Prompt urologic intervention also for obstruction of a solitary kidney, bilateral obstruction, AKI from obstruction, or refractory pain/vomiting
Percutaneous nephrostomy or ureteral stent for emergent decompression
Definitive treatment options: shock wave lithotripsy (SWL) for stones <2 cm, ureteroscopy with laser lithotripsy (preferred for distal/large stones), percutaneous nephrolithotomy (PCNL) for staghorn or >2 cm renal stones
Prevention: increase fluid intake to urine output ≥2.5 L/day FIRST for all stone types (the single most important measure); add targeted drugs (thiazide, potassium citrate, allopurinol) when a 24-h urine abnormality persists or stones recur. By stone type:
Calcium stones: high fluid intake (>2.5 L/day urine output), normal dietary calcium, low sodium, low animal protein, thiazide (HCTZ, chlorthalidone, indapamide) for hypercalciuria, potassium citrate for hypocitraturia
Uric acid stones: alkalinize urine (potassium citrate to pH 6.5-7), allopurinol if hyperuricosuria
Cystine stones: aggressive hydration, alkalinization, tiopronin or D-penicillamine for refractory
Struvite stones: complete stone removal + treat infection (urease inhibitor acetohydroxamic acid in select cases)
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