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Summary Table

Category	Key Features
Risk Factors	Male sex (3:1), age 30 to 50, family history, low fluid intake, high sodium/animal protein/oxalate diet, obesity, diabetes, hot climates, hyperparathyroidism, gout, Crohn disease, recurrent UTIs (struvite), medications (loop diuretics, topiramate, indinavir, acetazolamide)
Etiology	Supersaturation of stone-forming salts in urine leading to crystallization. Five main types: calcium oxalate (75 to 80%), calcium phosphate (10 to 15%), uric acid (5 to 10%), struvite (10%), cystine (1%)
Patient Presentation	Sudden-onset severe unilateral flank pain (renal colic) radiating to groin, testicle, or labia. Pain waxes and wanes. Nausea, vomiting, hematuria. Patient is restless and writhing, unable to lie still
Classic Physical Exam	Costovertebral angle (CVA) tenderness, patient in obvious distress moving constantly, absence of peritoneal signs, possible low-grade fever (red flag if high)
Key Diagnostic Results	Urinalysis showing hematuria (gross or microscopic) in ~85%, crystals on microscopy, urine pH clue (alkaline in struvite, acidic in uric acid). Noncontrast CT abdomen/pelvis is the gold standard. Ultrasound first in pregnancy
Management	Analgesia (NSAIDs first line, ketorolac IV/IM), IV fluids, antiemetics. Medical expulsive therapy with tamsulosin for distal ureteral stones 5 to 10 mm. Urgent decompression for obstruction with infection. SWL, ureteroscopy, or PCNL based on size and location
Keywords	"Worst pain of my life," "writhing in pain," "cannot get comfortable," "colicky flank pain radiating to groin," "tea-colored urine," "envelope-shaped crystals" (calcium oxalate), "coffin-lid crystals" (struvite), "hexagonal crystals" (cystine), "rhomboid crystals" (uric acid), "staghorn calculus"

1. Pathophysiology

Urolithiasis develops when urinary solutes exceed their solubility threshold, leading to supersaturation, crystallization, and aggregation into stones. The balance between promoters (calcium, oxalate, phosphate, uric acid, sodium) and inhibitors (citrate, magnesium) of crystallization governs stone formation. Low urine volume is the single most important modifiable risk factor across all stone types because it concentrates every solute simultaneously.

The colicky pain pattern reflects ureteral obstruction. As a stone migrates down the ureter, peristaltic contractions intensify against the obstruction, triggering wave-like pain that radiates along dermatomes T10 to L1. The pain pattern follows the stone's location: a stone at the ureteropelvic junction causes flank pain, mid-ureteral stones radiate to the lower abdomen and lateral flank, and ureterovesical junction (UVJ) stones cause groin/genital pain plus urinary urgency, frequency, and dysuria from bladder irritation. The three anatomic narrowings (UPJ, where the ureter crosses the iliac vessels, and UVJ) are the most common sites of impaction.

The classic restless presentation distinguishes renal colic from peritonitis. In peritonitis the patient lies perfectly still because movement worsens parietal peritoneal irritation, whereas a patient with renal colic paces, writhes, and shifts position trying to find relief. This single behavioral observation is a high-yield exam clue.

Stone composition reflects underlying metabolic or environmental drivers. Calcium oxalate stones form in normal to acidic urine and dominate overall epidemiology. Calcium phosphate stones favor alkaline urine and are linked to distal (type 1) renal tubular acidosis and primary hyperparathyroidism. Uric acid stones precipitate in acidic urine (pH less than 5.5), classically in patients with gout, tumor lysis syndrome, chronic diarrhea, and metabolic syndrome. Struvite (magnesium ammonium phosphate) stones form in alkaline urine generated by urease-producing organisms (Proteus mirabilis, Klebsiella, Staphylococcus saprophyticus, Pseudomonas), and these bacteria split urea into ammonia, raising pH and precipitating magnesium ammonium phosphate. Struvite stones grow rapidly into staghorn configurations filling the renal pelvis and calyces. Cystine stones occur in autosomal recessive cystinuria from defective tubular reabsorption of dibasic amino acids (cystine, ornithine, lysine, arginine; mnemonic COLA).

Hematuria results from mucosal abrasion as the stone traverses the urothelium. Its absence does not exclude stones because complete obstruction can prevent blood from reaching the bladder.

2. Classification

BASED ON LOCATION

Location of Stone	Most Prominent Manifestation
Renal calyx / renal pelvis	Often asymptomatic; may cause dull, intermittent flank pain. Hematuria (microscopic or gross) is the typical incidental finding.
Proximal ureter (UPJ to sacroiliac joint)	Severe colicky flank pain radiating to the ipsilateral costovertebral angle and upper abdomen. Nausea and vomiting are common due to shared autonomic innervation with the stomach and intestines (T11-L2 dermatome referral).
Mid ureter (crosses iliac vessels at the pelvic brim)	Colicky pain radiating from the flank toward the anterior abdomen and ipsilateral lower quadrant. May mimic appendicitis on the right or diverticulitis on the left.
Distal ureter (pelvic brim to UVJ)	Pain radiating to the ipsilateral groin, testicle (males), or labium (females). As the stone approaches the bladder, irritative voiding symptoms dominate: urgency, frequency, dysuria.
Bladder	Suprapubic pain or pressure, intermittent urinary stream, terminal hematuria, and positional dysuria (worsened by movement). May be asymptomatic if the stone is not obstructing the outlet.
Urethra	Acute urinary retention or severely decreased flow, dysuria, visible or palpable stone at the meatus (especially in males), urethral pain during voiding, and possible bloody urethral discharge. Most common in males due to longer urethra with natural narrowing at the fossa navicularis and external meatus.

BASED ON STONE TYPE

Stone Type	Frequency	Urine pH	Radiopacity on KUB	Crystal Shape	Key Associations
Calcium oxalate	75 to 80%	Variable (often acidic)	Radiopaque	Envelope or dumbbell	Idiopathic hypercalciuria, hyperoxaluria, low citrate, Crohn disease, ileal resection, ethylene glycol poisoning, vitamin C excess
Calcium phosphate	10 to 15%	Alkaline (greater than 6.5)	Radiopaque	Wedge-shaped prisms	Type 1 (distal) RTA, primary hyperparathyroidism
Uric acid	5 to 10%	Acidic (less than 5.5)	Radiolucent	Rhomboid or rosette	Gout, tumor lysis syndrome, chronic diarrhea, metabolic syndrome, high purine diet
Struvite (Mg-NH4-PO4)	10%	Alkaline (greater than 7.2)	Radiopaque	Coffin-lid	Urease-positive UTI (Proteus, Klebsiella, Pseudomonas, Staph saprophyticus); forms staghorn calculi; more common in women
Cystine	~1%	Acidic	Faintly radiopaque ("ground glass")	Hexagonal	Autosomal recessive cystinuria; presents in childhood/young adulthood; recurrent stones

3. Diagnostic Workup

Test	Role	Key Finding
Urinalysis	First line in every suspected case	Hematuria (~85%), pyuria, crystals, pH clue
Urine culture	Mandatory if pyuria/fever	Identifies causative organism, especially urease producers
Serum chemistry	Baseline	Creatinine, BUN, calcium, electrolytes
Noncontrast helical CT abdomen/pelvis	Gold standard / most accurate	Detects stones of all compositions, location, size, hydronephrosis
Renal ultrasound	First line in pregnancy, children, follow-up	Hydronephrosis, hydroureter, may miss small ureteral stones
KUB plain film	Adjunct, follow-up of known radiopaque stones	Misses radiolucent stones (uric acid, indinavir)
24-hour urine collection	Recurrent stone formers, after acute episode	Volume, calcium, oxalate, citrate, uric acid, sodium, pH
Stone analysis	After spontaneous passage or extraction	Definitive composition, guides prevention

The diagnostic sequence begins with urinalysis and a focused history. A patient with sudden colicky flank pain radiating to the groin, hematuria, and a restless presentation is highly suggestive of urolithiasis. Pregnancy must be excluded in women of reproductive age before imaging.

The best initial imaging test depends on the patient. In nonpregnant adults, noncontrast helical CT of the abdomen and pelvis is both the initial test of choice and the gold standard because it detects stones of all compositions (including radiolucent uric acid stones), measures size, identifies location, and reveals secondary signs such as hydronephrosis, perinephric stranding, and "tissue rim sign" around the stone. CT also evaluates alternative diagnoses (appendicitis, diverticulitis, abdominal aortic aneurysm).

In pregnant patients and children, ultrasound is the preferred initial study to avoid ionizing radiation. Ultrasound shows hydronephrosis and large stones at the UPJ or UVJ but has lower sensitivity for small mid-ureteral stones. If ultrasound is nondiagnostic in pregnancy and clinical suspicion remains high, MR urography is the next step.

KUB plain radiography has limited use because it misses radiolucent uric acid and indinavir stones and small stones overall. It is useful for following known radiopaque stones over time.

Urinalysis findings give compositional clues.

Alkaline urine (pH greater than 7) with positive nitrites and leukocyte esterase points to a struvite stone with infection.
Acidic urine (pH less than 5.5) without infection suggests uric acid
Hexagonal crystals are pathognomonic for cystine, and a positive sodium nitroprusside (cyanide-nitroprusside) test confirms cystinuria
Envelope-shaped (calcium oxalate dihydrate) or dumbbell-shaped (monohydrate) crystals point to calcium oxalate.

Indications for hospital admission and urgent urologic consultation include obstructing stone with fever or pyuria (urosepsis risk), acute kidney injury, intractable pain or vomiting, solitary or transplanted kidney, bilateral obstruction, and stones larger than 10 mm. Obstruction plus infection is a urologic emergency requiring immediate decompression.

For recurrent stone formers, a 24-hour urine collection (typically two collections off therapy) measures volume, calcium, oxalate, citrate, uric acid, sodium, potassium, magnesium, phosphate, creatinine, and pH. Combined with stone analysis, this guides targeted prevention.

4. Management & Treatment

Location	Stone Size	Preferred Therapy
Nephrolithiasis	< 5 mm	Observation, hydration, strain urine (~90% pass spontaneously)
	5 to 10 mm	Observation with tamsulosin 0.4 mg daily; reassess if persistent at 4 weeks
	< 2 cm	SWL (shock wave lithotripsy). Avoid in hard stones (cystine, CaOx monohydrate, brushite), obesity, pregnancy, coagulopathy
	> 2 cm or staghorn	PCNL (percutaneous nephrolithotomy)
Ureterolithiasis	< 5 mm	Observation, hydration, strain urine (~90% pass spontaneously)
	5 to 10 mm	Tamsulosin 0.4 mg daily (medical expulsive therapy); most effective for distal ureteral stones
	> 1 cm	Ureteroscopy with holmium laser lithotripsy (highest success rate for distal ureter). SWL is an option for proximal ureteral stones < 2 cm
	Any size + obstruction with infection	Emergency decompression (ureteral stent or percutaneous nephrostomy) + IV antibiotics. Do NOT attempt definitive removal during sepsis
Vesicolithiasis	Small to moderate	Transurethral cystolitholapaxy (endoscopic fragmentation and extraction)
	Large or very hard	Open or suprapubic cystolithotomy
	Any size	Must also address the underlying cause (BPH, neurogenic bladder, chronic retention), otherwise recurrence is likely

Acute Stabilization

The first priority in acute renal colic is pain control with NSAIDs, which work by reducing prostaglandin-mediated ureteral spasm and inflammation around the stone. Ketorolac 30 mg IV or IM is standard. NSAIDs outperform opioids for renal colic pain in head-to-head trials and avoid respiratory depression and nausea. Opioids remain useful adjuncts or alternatives when NSAIDs are contraindicated (renal failure, active GI bleed, third-trimester pregnancy). IV fluids restore euvolemia in vomiting patients but forced diuresis does not accelerate stone passage and may worsen pain. Antiemetics treat associated nausea.

The next best step in management logic after analgesia is risk stratification. Determine size, location, and presence of complications. Stones smaller than 5 mm in the distal ureter pass spontaneously in about 90% of cases, so outpatient management with oral analgesics, hydration, and urine straining (to capture the stone for analysis) is appropriate. Stones 5 to 10 mm benefit from medical expulsive therapy with tamsulosin 0.4 mg orally once daily, an alpha-1 blocker that relaxes ureteral smooth muscle and improves passage rates, particularly for distal ureteral stones. Treat for up to 4 weeks; if the stone has not passed, refer for urologic intervention.

Indications for Urgent Intervention

Refer for emergent urologic decompression in any of the following: urosepsis with obstructing stone, acute kidney injury, intractable pain or vomiting, solitary or transplanted kidney with obstruction, bilateral obstruction, or complete obstruction lasting more than a few days. Obstructed pyonephrosis is a true emergency: place a ureteral stent or percutaneous nephrostomy tube to decompress, start broad-spectrum IV antibiotics (such as piperacillin-tazobactam or a third-generation cephalosporin plus an aminoglycoside), and defer definitive stone removal until infection clears.

Definitive Stone Removal

Procedure choice depends on stone size, location, and composition:

Shock wave lithotripsy (SWL) uses focused acoustic waves to fragment stones for spontaneous passage. It is ideal for stones smaller than 2 cm in the kidney or proximal ureter. Contraindications include pregnancy, uncorrected coagulopathy, abdominal aortic aneurysm, and obstruction distal to the stone. SWL is less effective for hard stones (cystine, calcium oxalate monohydrate, brushite) and obese patients.

Ureteroscopy with holmium laser lithotripsy allows direct stone visualization and fragmentation. It has the highest success rate for distal ureteral stones and is the preferred procedure during pregnancy because it avoids ionizing radiation and shock waves.

Percutaneous nephrolithotomy (PCNL) is the procedure of choice for stones larger than 2 cm, staghorn calculi, and complex lower-pole stones. Access is gained through a flank puncture into the renal collecting system.

Long-Term Prevention by Stone Type

Universal advice for all stone formers: fluid intake to produce at least 2.5 liters of urine per day, reduce dietary sodium (less than 2,300 mg/day), maintain normal calcium intake (1,000 to 1,200 mg/day; calcium restriction worsens calcium oxalate stones by increasing oxalate absorption), and limit animal protein.

Stone Type	Targeted Prevention
Calcium oxalate	High fluids, low sodium, normal calcium, reduce oxalate-rich foods (spinach, nuts, chocolate, tea), thiazide diuretics for hypercalciuria, potassium citrate for hypocitraturia
Calcium phosphate	Treat underlying RTA or hyperparathyroidism; cautious use of citrate (raises urinary pH further)
Uric acid	Alkalinize urine with potassium citrate to pH 6 to 6.5, allopurinol if hyperuricosuria, low purine diet
Struvite	Complete surgical removal is essential, treat infection with culture-directed antibiotics, acetohydroxamic acid (urease inhibitor) in selected refractory cases
Cystine	High fluids (target greater than 3 liters/day), urinary alkalinization (pH greater than 7.5), thiol-binding agents (tiopronin, D-penicillamine) for refractory cases

Contraindications and Special Populations

In pregnancy, NSAIDs are contraindicated in the third trimester (premature ductus arteriosus closure) and best avoided throughout. Use acetaminophen and opioids for pain. Imaging starts with ultrasound. Tamsulosin is generally avoided. Ureteroscopy is the preferred intervention if needed.

In chronic kidney disease and elderly patients, NSAIDs can precipitate acute kidney injury and should be used with caution or avoided. Adjust opioid dosing.

5. Differential Diagnosis & Distractors

Differential Diagnosis	Why It Looks Similar	Key Discriminator
Pyelonephritis	Flank pain, CVA tenderness, hematuria possible, nausea	High fever, rigors, pyuria with bacteriuria, steady (not colicky) pain, patient lies still; CT shows no stone but may show perinephric stranding
Ruptured abdominal aortic aneurysm (AAA)	Sudden flank/back pain in older patient, can mimic colic	Age greater than 60, hypotension, pulsatile abdominal mass, syncope, known vascular disease; CT shows aneurysm with periaortic hematoma. Always consider in any older man with new "renal colic"
Ectopic pregnancy	Lower abdominal pain in woman of reproductive age, possible referred pain	Positive beta-hCG, vaginal bleeding, adnexal mass, hemodynamic instability if ruptured
Ovarian torsion	Acute unilateral pelvic pain with nausea/vomiting	Pain localized to pelvis, palpable adnexal mass, Doppler ultrasound shows absent ovarian flow
Appendicitis	Periumbilical pain migrating to right lower quadrant, may radiate to flank	Anorexia, fever, peritoneal signs (rebound, guarding), patient lies still, McBurney point tenderness
Diverticulitis	Left lower quadrant pain, fever, possible flank radiation	Older patient, bowel changes, peritoneal signs, CT shows pericolic inflammation
Biliary colic / cholecystitis	Colicky pain with nausea/vomiting	Right upper quadrant pain, postprandial after fatty meal, Murphy sign, gallstones on ultrasound
Musculoskeletal back pain	Flank pain	Reproducible with palpation/movement, no hematuria, normal urinalysis
Renal cell carcinoma	Hematuria, flank pain	Classically painless gross hematuria, weight loss, palpable mass; mass on imaging rather than stone
Herpes zoster (preeruptive phase)	Unilateral flank pain in dermatomal distribution	Pain precedes vesicular rash by 2 to 3 days; rash appears within days, dermatomal distribution

6. Traps & High-Yield Pearls

The classic exam trap in urolithiasis is failing to recognize "renal colic" as a ruptured abdominal aortic aneurysm in an older man. Any patient over 60, especially with hypertension or known vascular disease, presenting with new "first-time kidney stone" and flank or back pain should prompt immediate consideration of ruptured AAA. Test writers love this scenario because the cognitive shortcut to "stone" can kill the patient. Always check the abdomen for a pulsatile mass and pay attention to age and hemodynamics.

A second high-yield trap is missing obstruction with infection. A febrile patient with an obstructing stone has urosepsis until proven otherwise. The correct next step is urgent decompression (stent or percutaneous nephrostomy) plus IV antibiotics, not definitive stone removal. Choosing "ureteroscopy with stone extraction" instead of "decompression" is a classic distractor.

A third pitfall is counseling calcium restriction for calcium oxalate stones. Restricting dietary calcium increases free intestinal oxalate, raising urinary oxalate and worsening stone formation. The correct advice is normal calcium intake with reduced sodium and oxalate.

Other high-yield pearls:

The restless patient distinguishes renal colic from peritonitis. If the vignette says the patient is "writhing" or "cannot get comfortable," think stone; if the patient "lies perfectly still," think peritonitis.

Pregnancy changes imaging: ultrasound first, MRI if needed, avoid CT.

Tamsulosin works best for distal ureteral stones 5 to 10 mm; not indicated for stones smaller than 5 mm (which usually pass on their own) or larger than 10 mm (which usually need intervention).

Stone composition clues from urine pH: alkaline points to struvite or calcium phosphate; acidic points to uric acid or cystine. Crystal shapes are buzzwords: hexagonal equals cystine, coffin-lid equals struvite, envelope or dumbbell equals calcium oxalate, rhomboid equals uric acid.

Staghorn calculi are struvite until proven otherwise, and complete surgical removal is required because residual fragments harbor bacteria and cause recurrent infection. Treat the infection but understand that antibiotics alone will not clear the stone.

Uric acid stones are radiolucent on KUB but visible on noncontrast CT. Treatment with urinary alkalinization (potassium citrate) can dissolve them, the only stone type that can be medically dissolved without surgery.

Cystinuria presents in childhood or young adulthood with recurrent stones, autosomal recessive inheritance, and hexagonal crystals. The sodium nitroprusside (cyanide-nitroprusside) test is the screening test.

The core competencies tested in urolithiasis questions are: recognizing the classic vignette, sequencing imaging correctly (CT in adults, ultrasound in pregnancy), choosing the right initial pharmacologic management (NSAIDs, then tamsulosin for eligible stones), identifying emergencies (obstruction with infection, ruptured AAA mimic), and matching stone type to urine pH, crystal shape, and prevention strategy.

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