NAVLE Urinary System High-Yield Guide: Kidney & Bladder Disease Decoded

Urinary disease appears on every NAVLE sitting — expect 15 to 25 questions spanning dogs, cats, horses, ruminants, and small mammals. The examiners test the same core concepts repeatedly: the acute kidney injury versus chronic kidney disease distinction, IRIS staging thresholds, species-specific urolith types, feline lower urinary tract disease, and the protein-losing nephropathies. Master these five pillars and you will convert a historically weak area into a reliable source of correct answers.

This guide covers veterinary nephrology NAVLE high-yield content in depth — clinical signs, diagnostics, staging, treatment, and prevention — organized the way the exam tests it.

AKI vs CKD: The Most Tested Distinction in NAVLE Urinary Questions

Acute kidney injury and chronic kidney disease share many clinical and laboratory features but require completely different management. The NAVLE will give you a case and expect you to classify it correctly before asking about treatment.

AKI is a sudden, often reversible decline in glomerular filtration rate. Because the kidneys were normal before the insult, the body has had no time to compensate — azotemia rises rapidly, and animals frequently present in a uremic crisis. Kidney size is normal to enlarged on imaging. With rapid and aggressive intervention, a meaningful proportion of patients recover full or near-full function.

CKD results from irreversible progressive nephron loss. More than 75% of nephron mass must be destroyed before azotemia becomes measurable — this is why SDMA (symmetric dimethylarginine) has become valuable as an earlier marker. Kidneys are typically small and irregular on imaging. CKD is managed, not cured.

Common Causes of AKI — High-Yield List

The NAVLE loves nephrotoxin questions. Know these by species:

• NSAIDs — inhibit prostaglandin-mediated afferent arteriolar dilation; dangerous in dehydrated or hypotensive patients; all species
• Aminoglycosides (gentamicin, amikacin) — proximal tubular necrosis; dose-dependent; monitor with urinalysis for casts
• Lily toxicity (cats only) — true lilies (Lilium, Hemerocallis); profound AKI within 24-72 hours; decontamination + aggressive IV fluids early
• Grape and raisin toxicity (dogs) — mechanism unknown; idiosyncratic; any dose can cause severe AKI
• Ethylene glycol — oxalate crystal deposition in tubules; calcium oxalate monohydrate crystals in urine; antidote is fomepizole (4-MP) in dogs, ethanol in cats; treat within hours
• Contrast dye — contrast-induced nephropathy; pre-hydrate patients
• Leptospira — covered in depth below
• Ischemia — prolonged hypotension (anesthesia, shock, GDV, trauma)

AKI treatment: Aggressive IV fluid diuresis is the cornerstone. Furosemide can convert oliguric to non-oliguric AKI. Low-dose dopamine was historically used for renal vasodilation but evidence is weak; it is still tested. Correct electrolyte abnormalities (hyperkalemia is life-threatening). Dialysis is available at referral centers for severe cases.

Canine CKD: IRIS Staging and Management

The International Renal Interest Society (IRIS) staging system is the standard framework for classifying and managing CKD in dogs and cats. Expect direct NAVLE kidney disease staging questions.

IRIS CKD Staging — Dogs and Cats

IRIS staging also includes substages based on blood pressure and proteinuria:

• Proteinuria substage: UPC <0.2 = non-proteinuric; 0.2–0.5 = borderline; >0.5 = proteinuric (treat if persistent)
• Blood pressure substage: normotensive (<140 mmHg systolic); pre-hypertensive (140–159); hypertensive (160–179); severely hypertensive (>180)

Key SDMA point for the NAVLE: SDMA >14 ?g/dL indicates approximately 40% loss of GFR — it becomes elevated before creatinine increases above the reference range (creatinine requires ~75% nephron loss). This makes SDMA the earlier biomarker for Stage 1 CKD.

Canine CKD Management by Stage

Staged management is high-yield for NAVLE CKD questions:

• Phosphorus restriction: renal diet (phosphorus <0.4% DM in dogs) + phosphate binders if diet alone is insufficient. Binders include aluminum hydroxide (cheap, effective; risk of aluminum toxicity long-term), lanthanum carbonate, and calcium-based binders (avoid if hypercalcemic)
• Protein restriction: moderate restriction once azotemic (Stage 2+); do not restrict protein in nonazotemic animals — it causes muscle wasting without benefit
• Omega-3 fatty acids: reduce glomerular hypertension and slow progression
• Calcitriol (active vitamin D): CKD impairs renal 1-alpha-hydroxylase ? calcitriol deficiency ? secondary renal hyperparathyroidism (CKD-MBD). Supplementing calcitriol suppresses PTH and may slow progression. Monitor calcium carefully — hypercalcemia worsens CKD
• Telmisartan: AT1 angiotensin receptor blocker approved for feline CKD proteinuria; increasingly used off-label in dogs
• Benazepril (ACE inhibitor): reduces proteinuria in dogs with CKD; reduces intraglomerular pressure
• Erythropoietin / darbepoetin: for nonregenerative anemia of CKD (reduced EPO production). Use darbepoetin alfa — less immunogenic than recombinant human EPO. Monitor hematocrit and blood pressure
• Amlodipine (calcium channel blocker): first-line antihypertensive in cats; second-line in dogs
• Anti-nausea therapy: maropitant, ondansetron, famotidine for uremic gastritis

Feline CKD: Species-Specific Considerations

Feline CKD is extremely common — approximately 30–40% of cats over 10 years of age are affected. The NAVLE will test feline-specific nuances.

IRIS staging thresholds differ for cats (see table above). Notably, feline creatinine reference ranges are slightly higher than dogs, reflecting lower muscle mass relative to kidney size.

SDMA in cats: SDMA >14 ?g/dL is the Stage 1 indicator. Because cats mask illness, SDMA elevation during routine wellness testing is often the first sign of early CKD.

CKD-Mineral Bone Disorder (CKD-MBD) in Cats

CKD impairs phosphorus excretion ? hyperphosphatemia ? stimulates PTH (secondary renal hyperparathyroidism) ? bone resorption and soft tissue mineralization. Simultaneously, reduced calcitriol synthesis reduces calcium absorption. Together, this triad constitutes CKD-MBD. Treatment targets:

1. Restrict dietary phosphorus (renal diet)
2. Add phosphate binders if renal diet alone insufficient
3. Supplement calcitriol once hyperphosphatemia is controlled

Telmisartan in Cats

Telmisartan (Semintra) is the only drug licensed specifically for reduction of proteinuria associated with CKD in cats. It is an AT1 receptor blocker (ARB). Mechanism: blocks angiotensin II at the AT1 receptor ? reduces efferent arteriolar constriction ? lowers intraglomerular pressure ? reduces protein filtration. The NAVLE will test this drug by name — know its class, indication, and species.

Subcutaneous Fluids for Cats with CKD

Home subcutaneous fluid therapy is a practical cornerstone of feline CKD management in Stages 3 and 4. Owners administer 0.9% NaCl or lactated Ringer's solution subcutaneously 2–3 times weekly. It improves hydration, reduces azotemia, and extends quality of life. The NAVLE may present a management question asking the best at-home supportive measure — subcutaneous fluids is the correct answer for moderate-to-severe feline CKD.

Feline Lower Urinary Tract Disease (FLUTD)

FLUTD is a clinical syndrome — hematuria, dysuria, pollakiuria, periuria — and the NAVLE will test your ability to diagnose the underlying cause, not just recognize the signs.

FLUTD Causes by Frequency

• Feline idiopathic cystitis (FIC): 55–60% of FLUTD cases — the most common cause. Diagnosis of exclusion. Stress is the key trigger. Multimodal environmental modification (MEMO) is the cornerstone of management: Feliway pheromone diffusers, puzzle feeders, increased water intake (wet food), hiding spaces, litter box optimization, reduced inter-cat tension
• Urolithiasis: struvite and calcium oxalate together account for ~80% of feline uroliths
• Urethral plugs: soft matrix plugs (mineral + mucus) in male cats — distinct from true uroliths; dissolved with catheterization and diet change
• Urinary tract infection: uncommon in young cats (<5%); more common in older cats with CKD, diabetes, or prior instrumentation
• Anatomic defects, neoplasia: less common; consider in older cats with refractory signs

Urethral Obstruction in Male Cats — Emergency

Urethral obstruction (UO) is a life-threatening emergency unique to male cats (narrow penile urethra). Clinical signs: straining with no urine production, crying, painful distended bladder, collapse. If untreated: post-renal azotemia ? hyperkalemia ? fatal arrhythmia.

Emergency management steps:

1. 1Stabilize: IV catheter, IV fluids, ECG monitoring. Treat hyperkalemia if arrhythmia present (calcium gluconate, sodium bicarbonate, dextrose/insulin)
2. 2Sedate and deobstruct: retrohydropulsion with saline and urethral catheter passage; relieve obstruction
3. 3Indwelling catheter: maintain for 24-48 hours while post-obstructive diuresis is managed with IV fluid support
4. 4Monitor electrolytes and azotemia: creatinine and potassium should normalize within 48-72 hours if no underlying CKD
5. 5Long-term prevention: wet food, water fountains, stress reduction, canned urinary diet. Recurrent blockers: consider perineal urethrostomy (PU) — widens the urethral opening; curative for obstruction but does not prevent cystitis

Urolith Types by Species: The High-Yield NAVLE Chart

Urolith type determines treatability by dissolution (some dissolve; most do not), prevention strategy, and underlying risk factors. This is a reliable source of multiple NAVLE questions per sitting.

Canine Lower Urinary Tract Disease: UTI, Uroliths, and USMI

Dogs develop urinary tract infections far more commonly than cats. Escherichia coli is the most common uropathogen, followed by Staphylococcus pseudintermedius, Proteus, Klebsiella, and Enterococcus. Culture and sensitivity is essential — empirical antibiotic selection is reasonable for uncomplicated first infections, but culture is mandatory for recurrent, complicated, or hospital-acquired UTIs.

Urethral Sphincter Mechanism Incompetence (USMI)

USMI is the most common cause of urinary incontinence in dogs — specifically in spayed female dogs of medium to large breeds. The mechanism is a reduction in urethral tone following ovariectomy (estrogen withdrawal). Dogs leak urine while relaxed or sleeping.

Diagnosis: rule out ectopic ureter (congenital; younger patients; continuous dribbling), UTI, detrusor instability. Urethral pressure profilometry is the definitive test but rarely needed clinically.

USMI treatment options:

• Phenylpropanolamine (PPA): alpha-1 adrenergic agonist; increases urethral tone; first-line medical therapy; response rate ~85%; administer twice daily; monitor for hypertension
• Estriol (Incurin): licensed estrogen supplement in some countries; restores urethral receptor density; second-line or combination therapy
• Colposuspension / urethropexy: surgical option for medically refractory cases; repositions the urethra into the abdominal pressure zone

Leptospirosis: AKI from an Infectious Cause

Leptospirosis is both a high-yield infectious disease topic and a key cause of AKI in dogs. Leptospira interrogans serovars cause disease in dogs through contact with contaminated water or wildlife urine.

Pathogenesis: Leptospires penetrate mucous membranes ? bacteremia ? renal tubular colonization ? acute tubular necrosis ? AKI. Hepatitis occurs concurrently in some serovars (icterohaemorrhagiae, canicola).

Clinical signs: acute onset vomiting, lethargy, polyuria/oliguria, jaundice (if hepatic involvement), uveitis, pulmonary hemorrhage (Leptospira pulmonary hemorrhage syndrome — LPHS, high mortality).

Diagnosis: Microscopic agglutination test (MAT) — paired titers (4-fold rise confirms active infection); PCR on blood (early) or urine (later); in-house SNAP test (screening).

Treatment: Penicillin or ampicillin for initial bacteremia phase; followed by doxycycline (2 weeks) to eliminate leptospiruria and prevent carrier state. IV fluids for AKI support. Strict barrier precautions — zoonotic risk to staff.

Vaccination: L4 vaccine covers four serovars (canicola, icterohaemorrhagiae, grippotyphosa, pomona). Annual vaccination for at-risk dogs. The NAVLE will ask which serovars are covered and the zoonotic significance.

Equine Urinary Disease

Horses present with urinary disease less commonly than small animals, but the NAVLE tests a predictable set of scenarios.

Equine AKI

Causes include NSAID toxicity (phenylbutazone — "bute"; proximal tubular damage, especially with dehydration), aminoglycoside nephrotoxicity, and hemoglobinuric nephropathy from rhabdomyolysis or acute hemolysis. Treatment is supportive fluid diuresis and removal of the offending drug.

Sabulous Urolithiasis in Horses

Sabulous urolithiasis refers to accumulation of calcium carbonate sand/sludge in the equine bladder — most common in mares (relaxed urethral sphincter allows retrograde accumulation). Signs: hematuria, stranguria, recumbency. Diagnosis: rectal palpation or endoscopy. Treatment: bladder lavage and siphoning under sedation. True uroliths in horses are calcium carbonate; they form in the alkaline equine urine and are not amenable to medical dissolution — surgical cystotomy is required for large calculi.

Equine CKD

Horses with CKD show weight loss, poor haircoat, hypercalcemia (horses normally excrete calcium in urine — CKD impairs this ? hypercalcemia and calcinosis), and ventral edema. Diagnosis by azotemia, urinalysis (isosthenuric urine), and elevated fractional excretion of sodium.

Bovine and Ruminant Urinary Disease

Obstructive urolithiasis in small ruminants (wethers, castrated male goats and sheep) is one of the highest-yield ruminant topics on the NAVLE. The combination of a high-grain diet (high phosphorus), low water intake, and the anatomy of the castrated male ruminant (narrow sigmoid flexure and urethral process) creates a perfect storm for struvite and calcium apatite urolith formation.

Clinical signs: straining to urinate, kicking at abdomen, vocalizing, extended penis with crystalline deposits at the urethral process, eventual bladder rupture or urethral perforation.

Treatment options (in order of increasing intervention):

1. Urethral process amputation: removes the narrowest point; often provides temporary relief; obstruction may recur at sigmoid flexure
2. Perineal urethrostomy: bypasses the sigmoid flexure; creates a permanent stoma; allows voiding but not breeding; definitive for pets/non-breeding animals
3. Tube cystostomy: surgical bladder drainage while attempting dissolution and recovery; option when urethrostomy is not performed
4. Bladder rupture management: abdominocentesis to relieve uroperitoneum + surgical repair

Prevention: dietary calcium to phosphorus ratio >2:1; ammonium chloride as a urinary acidifier; encourage water intake (salt supplementation); delay castration to allow urethral development.

Glomerular Disease and Protein-Losing Nephropathy

Glomerulonephritis and amyloidosis are the two primary causes of protein-losing nephropathy (PLN) in dogs and cats. The NAVLE tests the urine protein:creatinine (UPC) ratio, its interpretation, and when to treat.

UPC Ratio Interpretation

• Normal: <0.2 in dogs, <0.4 in cats
• Borderline: 0.2–0.5 (dogs); 0.2–0.4 (cats)
• Proteinuric: >0.5 (treat if persistent and glomerular origin confirmed)

Always collect urine by cystocentesis for accurate UPC; confirm on 2–3 samples before diagnosing persistent proteinuria. Rule out pre-renal (hemoglobin, myoglobin) and post-renal (hemorrhage, inflammation) causes.

Causes of Glomerulonephritis

Immune complex deposition is the most common mechanism. Underlying triggers include: chronic infectious disease (ehrlichiosis, heartworm, Lyme disease, leishmaniasis, FeLV, FIV), chronic inflammatory disease (IBD, neoplasia), and idiopathic. Amyloidosis occurs in Shar-Peis (renal amyloidosis) and Abyssinian cats (familial amyloidosis).

Treatment of PLN

• Benazepril or enalapril (ACE inhibitors): reduce intraglomerular pressure and proteinuria; first-line in dogs
• Telmisartan: first-line in cats; can combine ARB + ACEi in refractory cases
• Amlodipine: for concurrent hypertension (especially cats)
• Omega-3 fatty acids: anti-inflammatory, renoprotective
• Anticoagulation: consider aspirin or clopidogrel — PLN causes loss of antithrombin III ? hypercoagulable state ? thromboembolic disease risk
• Treat underlying cause (antibiotics for infectious, immunosuppressives for immune-mediated glomerulonephritis)

For small ruminant urolithiasis and bovine renal disease, also see the NAVLE Canine High-Yield Guide and species-specific articles linked at the end of this article.

NAVLE Urinary Questions: Rapid Review

Before the FAQ, here is a quick-reference summary of the most testable NAVLE urinary high-yield facts:

• AKI = reversible, sudden onset, normal/enlarged kidneys; CKD = irreversible, progressive, small kidneys
• SDMA >14 ?g/dL = IRIS Stage 1 CKD (earlier than creatinine rises)
• Lily toxicity = cats only ? AKI; grapes/raisins = dogs ? AKI; ethylene glycol antidote = fomepizole in dogs
• FIC (idiopathic cystitis) = most common FLUTD cause in cats (55–60%); treat with MEMO
• Male cat urethral obstruction = emergency; manage hyperkalemia before deobstruction
• Struvite in dogs = infection-induced by Staphylococcus; dissolvable
• Calcium oxalate = cannot be dissolved in any species; surgical or lithotripsy
• Urate uroliths = Dalmatians + PSS dogs; allopurinol + low-purine diet
• USMI = spayed females; PPA first-line
• Leptospirosis = AKI + hepatitis + zoonotic; penicillin then doxycycline; L4 vaccine
• UPC >0.5 = proteinuric; treat with ACE inhibitor (dogs) or telmisartan (cats)
• Wether obstructive urolithiasis: urethral process amputation ? perineal urethrostomy; prevent with Ca:P >2:1 + ammonium chloride
• Telmisartan = AT1 receptor blocker, licensed for feline CKD proteinuria
• Calcitriol = active vitamin D; supplement in CKD to suppress secondary hyperparathyroidism

Frequently Asked Questions: NAVLE Kidney Disease and Urinary Questions

Related NAVLE Study Guides

Urinary disease overlaps heavily with internal medicine, infectious disease, and species-specific content. Continue building your NAVLE veterinary nephrology knowledge with these related guides:

• NAVLE Exam Complete Guide — full exam overview, question distribution, and study strategy
• NAVLE Canine High-Yield Guide — leptospirosis, renal disease in context of canine internal medicine
• NAVLE Feline High-Yield Guide — feline CKD, FLUTD, hyperthyroidism interactions with renal disease
• NAVLE Diagnostics High-Yield Guide — urinalysis interpretation, UPC ratio, sediment analysis, renal biopsy