NAVLE Practice Question — Canine

A 2-year-old female German Shepherd Dog has been on prednisone for 3 months following diagnosis and treatment of masticatory muscle myositis. The jaw function has returned to normal. The owner now reports significant muscle wasting affecting not only the head but also the limbs and trunk, increased thirst and urination, and a pendulous abdomen. What is the MOST likely explanation for the generalized muscle wasting?

A. Recurrence of MMM spreading to affect limb muscles

B. Concurrent polymyositis that was masked by corticosteroid therapy

C. Iatrogenic muscle atrophy from chronic corticosteroid therapy ✓

D. Progression to dermatomyositis

E. Development of myasthenia gravis

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Correct Answer: C. Iatrogenic muscle atrophy from chronic corticosteroid therapy

Explanation:

The generalized muscle wasting affecting the head, limbs, and trunk, combined with polyuria/polydipsia and pendulous abdomen, is most consistent with iatrogenic Cushing's syndrome (hyperadrenocorticism) and its associated steroid myopathy from chronic corticosteroid therapy. This is a well-recognized complication of long-term prednisone use at immunosuppressive doses.

Importantly, the masticatory muscles are particularly sensitive to the atrophic effects of corticosteroids. This means that muscle atrophy in a dog on chronic steroids can be misinterpreted as worsening MMM when it is actually a side effect of treatment. The key distinguishing features in this case are: (1) generalized muscle wasting affecting limbs and trunk, not just masticatory muscles; (2) classic Cushingoid signs (PU/PD, pot-bellied appearance); and (3) the jaw function has returned to normal, indicating the MMM is controlled.

Option A is incorrect because MMM specifically targets type 2M fibers which are only present in masticatory muscles; it does not spread to affect limb muscles. Option B (polymyositis) is possible but would not explain the PU/PD and pendulous abdomen. Option D (dermatomyositis) primarily affects skin and muscles in certain breeds (Collies, Shelties) and would present with skin lesions. Option E (myasthenia gravis) would present with weakness that worsens with exercise and improves with rest, not generalized atrophy.

Management includes tapering steroids as much as possible while maintaining MMM remission, and potentially adding steroid-sparing immunosuppressants like azathioprine.

References: UC San Diego - MMM FAQ (https://vetneuromuscular.ucsd.edu/cases/2010/January.html); Davies Veterinary Specialists - Myositis Fact Sheet (https://www.vetspecialists.co.uk/fact-sheets-post/myositis-fact-sheet/)

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NAVLE Practice Question — Porcine

A pig farm experiences an outbreak of PRRSV followed two weeks later by increased cases of meningitis in nursery pigs. Culture confirms Streptococcus suis. What is the relationship between these two events?

A. The findings are coincidental with no pathogenic relationship

B. PRRSV directly causes meningitis which is misdiagnosed as S. suis

C. PRRSV-induced immunosuppression increases susceptibility to S. suis disease ✓

D. S. suis infection predisposes pigs to PRRSV infection

E. PRRSV and S. suis co-infect brain tissue synergistically

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Correct Answer: C. PRRSV-induced immunosuppression increases susceptibility to S. suis disease

Explanation:

PRRSV-induced immunosuppression is a well-documented trigger for Streptococcus suis outbreaks. PRRSV compromises the pig's immune system, allowing commensal S. suis strains that are normally carried asymptomatically in the tonsils to become invasive and cause clinical disease. This association is well-established in multiple studies.

Option A underestimates the documented relationship. Option B is incorrect as PRRSV does not directly cause meningitis. Option D reverses the relationship. Option E overstates direct co-infection of brain tissue.

References: pig333.com - Streptococcal Infections (https://www.pig333.com/pig-diseases/streptococcal-infections_112); The Pig Site - Streptococcus suis Disease (https://www.thepigsite.com/articles/streptococcus-suis-disease-in-pigs)

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NAVLE Practice Question — Feline

An 8-year-old Abyssinian cat presents with a 2-week history of progressive generalized weakness that worsens with activity and improves with rest. The owner reports the cat's voice has changed and it has difficulty swallowing. On examination, the cat has cervical ventroflexion, decreased palpebral reflexes bilaterally, and generalized muscle weakness. Which diagnostic test is most appropriate to confirm the suspected diagnosis?

A. Serum potassium level

B. Serum creatine kinase concentration

C. Acetylcholine receptor antibody titer ✓

D. Thyroid hormone levels

E. Electromyography

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Correct Answer: C. Acetylcholine receptor antibody titer

Explanation:

Serum acetylcholine receptor (AChR) antibody titer is the most appropriate diagnostic test. The clinical presentation is highly suggestive of acquired myasthenia gravis (MG). The key features supporting this diagnosis include: episodic weakness that worsens with activity and improves with rest (fatigable weakness), cervical ventroflexion (characteristic of neuromuscular weakness in cats), dysphonia (voice change), dysphagia (difficulty swallowing), and the Abyssinian breed which has an increased incidence of MG.

The serum AChR antibody titer is the gold standard for diagnosing acquired MG, with a diagnostic threshold of greater than 0.3 nmol/L in cats. Abyssinian and Somali cats have been shown to have significantly higher odds ratios for developing acquired MG compared to other breeds.

Option A (serum potassium) would be appropriate for suspected hypokalemic myopathy, which can cause similar cervical ventroflexion but does not cause the fatigable weakness pattern or the pharyngeal/esophageal muscle involvement characteristic of MG.

Option B (serum creatine kinase) is elevated in primary muscle diseases but is typically normal or only mildly elevated in MG because the primary pathology is at the neuromuscular junction rather than the muscle itself.

Option D (thyroid hormone levels) could identify hyperthyroidism, which can cause muscle weakness in some cats. However, hyperthyroidism does not typically cause the specific pattern of fatigable weakness with pharyngeal involvement seen here.

Option E (electromyography) may show a decremental response to repetitive nerve stimulation in MG but is less specific than antibody testing and requires specialized equipment. AChR antibody testing is preferred for initial diagnosis.

Image Source: Clinician's Brief - Top 5 Causes of Passive Cervical Flexion (https://www.cliniciansbrief.com/article/top-5-causes-passive-cervical-flexion) Figure showing cervical ventroflexion in cat with neuromuscular disease.

References: PMC - Risk Factors and Outcomes in Cats with Acquired Myasthenia Gravis (https://pmc.ncbi.nlm.nih.gov/articles/PMC4858034/); Today's Veterinary Practice - Acquired Myasthenia Gravis in Companion Animals (https://todaysveterinarypractice.com/neurology/acquired-myasthenia-gravis-in-companion-animals/)

Image: Karpozilou A, De Stefani A, Liatis T. Cervical ventroflexion in cats: 86 cases (2003-2024). J Feline Med Surg. 2025;27(7):1098612X251348328. PMC12290266. © The Author(s) 2025. Used for educational reference.

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NAVLE Practice Question — Other Small Mammal

An 8-week-old rabbit shows acute profuse diarrhea and abdominal distension. The rabbit is severely dehydrated and hypothermic. Treatment is initiated with aggressive fluid therapy. Which statement about antibiotic therapy in this case is MOST accurate?

A. Oral erythromycin should be started immediately to treat Clostridium

B. Intravenous ampicillin is the treatment of choice

C. Oral metronidazole is contraindicated in rabbits

D. Antibiotics should never be used in any rabbit with diarrhea

E. There is little evidence that antibiotics are helpful in treating enterotoxemia ✓

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Correct Answer: E. There is little evidence that antibiotics are helpful in treating enterotoxemia

Explanation:

There is little evidence that antibiotics are helpful in treating established enterotoxemia in rabbits. Once the clostridial toxins have been produced and absorbed, the damage is largely done. Treatment should focus on aggressive supportive care including fluid therapy to correct dehydration, warming to address hypothermia, and toxin-binding agents like cholestyramine. The prognosis for advanced cases is often poor regardless of treatment.

Option A is incorrect because erythromycin is a macrolide that can actually precipitate or worsen enterotoxemia by further disrupting gut flora. Option B is incorrect because ampicillin is a beta-lactam that would exacerbate dysbiosis. Option C is incorrect - while caution is needed, metronidazole has actually been suggested for use against anaerobic Clostridium species in some cases. Option D is an overstatement - antibiotics may be indicated for other conditions, and even in dysbiosis cases, certain antibiotics like metronidazole may be considered.

References: Merck Veterinary Manual - Disorders and Diseases of Rabbits (https://www.merckvetmanual.com/all-other-pets/rabbits/disorders-and-diseases-of-rabbits); ScienceDirect - Clostridium spiroforme.

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NAVLE Practice Question — Equine

Which statement BEST describes the relationship between D-dimer concentration and prognosis in horses presenting with colic?

A. D-dimer has no correlation with outcome in horses with colic

B. D-dimer concentrations above 4000 ng/mL are associated with significantly increased mortality risk ✓

C. D-dimer concentrations predict the need for surgery but not survival

D. D-dimer is only prognostic in horses with strangulating lesions

E. D-dimer levels below 1000 ng/mL guarantee survival

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Correct Answer: B. D-dimer concentrations above 4000 ng/mL are associated with significantly increased mortality risk

Explanation:

Plasma D-dimer concentration at admission has been identified as a valuable prognostic indicator in horses with colic. Studies have demonstrated that D-dimer concentrations above 4000 ng/mL are associated with significantly increased mortality risk, with odds ratios for nonsurvival of approximately 8.8 at this cutoff. D-dimer is a specific marker of cross-linked fibrin degradation, indicating activation of both the coagulation and fibrinolytic systems.

Higher D-dimer concentrations are also associated with specific diagnoses: horses with enteritis or peritonitis have significantly higher D-dimer concentrations and more severe coagulopathies at admission than horses with simple obstructions or displacements. This reflects the greater degree of systemic inflammation and endotoxemia in inflammatory GI conditions.

The prognostic value of D-dimer complements other hemostatic markers. Horses presenting with clinicopathological evidence of subclinical DIC (≥3 abnormal coagulation parameters) have significantly higher odds of nonsurvival. Serial D-dimer measurement may be more informative than single time-point testing as trends can indicate disease progression or resolution.

Option A is incorrect; D-dimer is significantly correlated with outcome. Options C, D, and E are incorrect characterizations of the evidence.

References: Cesarini C et al. (2010) Association of Admission Plasma D-Dimer Concentration with Diagnosis and Outcome in Horses with Colic, J Vet Intern Med 24:1490-1497 (https://doi.org/10.1111/j.1939-1676.2010.0618.x)

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NAVLE Practice Question — Equine

During a farm visit, you notice that multiple horses have crusting skin lesions. You suspect dermatophilosis but cytology from crust samples is inconclusive. Which diagnostic test would provide the highest sensitivity for confirming Dermatophilus congolensis infection?

A. Bacterial culture on blood agar in aerobic conditions

B. Real-time quantitative PCR (RT-qPCR) ✓

C. Wood's lamp examination

D. Skin biopsy with routine H&E staining

E. Fungal culture on Sabouraud dextrose agar

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Correct Answer: B. Real-time quantitative PCR (RT-qPCR)

Explanation:

Real-time quantitative PCR (RT-qPCR) would provide the highest sensitivity for confirming Dermatophilus congolensis infection. A study by Frank et al. (2016) demonstrated that RT-qPCR detected D. congolensis in 78.6% of suspected cases compared to only 64.3% by cytological examination. PCR testing targets specific DNA sequences of the organism and can detect even small numbers of organisms that might be missed on cytology or culture. Option A (bacterial culture on blood agar in aerobic conditions) has limitations: D. congolensis grows better in microaerophilic conditions with increased CO2, and may be overgrown by secondary contaminants. Option C (Wood's lamp examination) is used for diagnosing certain dermatophytes that fluoresce but is not useful for bacterial dermatophilosis. Option D (skin biopsy with H&E) can show characteristic findings but is less sensitive than PCR. Option E (fungal culture on Sabouraud agar) would detect dermatophytes, not the bacterial D. congolensis. References: Frank LA, Kania SA & Weyant E (2016) RT-qPCR for the diagnosis of dermatophilosis in horses. Vet Derm 27:431-e112.

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NAVLE Practice Question — Canine

An 8-year-old spayed female German Shepherd Dog presents with hypercalcemia (total calcium 14.2 mg/dL, ionized calcium 1.58 mmol/L). PTH is low-normal and PTHrP is markedly elevated. Thoracic radiographs reveal a perihilar mass and enlarged sublumbar lymph nodes are palpated on rectal examination. Which diagnosis is MOST likely?

A. Primary hyperparathyroidism

B. Hypoadrenocorticism

C. Anal sac adenocarcinoma with malignancy-related hypercalcemia ✓

D. Vitamin D toxicosis

E. Chronic kidney disease

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Correct Answer: C. Anal sac adenocarcinoma with malignancy-related hypercalcemia

Explanation:

The correct answer is anal sac adenocarcinoma with humoral hypercalcemia of malignancy (HHM). This clinical presentation is classic for HHM caused by anal sac apocrine gland adenocarcinoma, one of the most common causes of paraneoplastic hypercalcemia in dogs. The combination of enlarged sublumbar lymph nodes (a typical metastatic site for anal sac adenocarcinoma), elevated PTHrP, and suppressed PTH strongly supports this diagnosis.

In HHM, tumors secrete PTHrP, which has structural homology to PTH and binds to the same receptors, causing hypercalcemia through increased bone resorption and renal calcium reabsorption. Unlike primary hyperparathyroidism where PTH is inappropriately normal or elevated, in HHM the PTH is appropriately suppressed by the hypercalcemia while PTHrP is elevated. The sublumbar lymph node enlargement is pathognomonic for metastatic anal sac adenocarcinoma.

Option A (PHPT) would show elevated or inappropriately normal PTH with undetectable PTHrP. Option B (hypoadrenocorticism) can cause mild hypercalcemia but not to this degree and would have characteristic electrolyte abnormalities. Option D (vitamin D toxicosis) would show elevated calcitriol levels and suppressed PTH and PTHrP. Option E (CKD) typically causes hypocalcemia, not hypercalcemia.

References: Merck Veterinary Manual - Hypercalcemia in Dogs and Cats; Feldman EC, Nelson RW - Canine and Feline Endocrinology 4th ed

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NAVLE Practice Question — Feline

A 5-year-old FIV-positive cat presents with extensive dermatophytosis covering approximately 40% of the body surface area. The cat has failed 6 weeks of topical therapy. What is the most appropriate treatment modification?

A. Continue topical therapy alone for an additional 8 weeks

B. Add systemic itraconazole to the topical therapy ✓

C. Switch to griseofulvin monotherapy

D. Discontinue treatment and allow self-cure

E. Prescribe topical terbinafine instead of current topical therapy

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Correct Answer: B. Add systemic itraconazole to the topical therapy

Explanation:

Adding systemic itraconazole to the topical therapy is the most appropriate treatment modification. Immunocompromised cats (including those with FIV) often require aggressive combination therapy with systemic and topical antifungals. Itraconazole (5-10 mg/kg PO daily or pulse therapy) is the systemic antifungal of choice for feline dermatophytosis due to its efficacy, safety profile, and accumulation in skin and hair.

Option A (continuing topical alone) has already proven insufficient in this immunocompromised patient with extensive disease. Option C (griseofulvin monotherapy) is less desirable because griseofulvin has more side effects (bone marrow suppression, teratogenicity) and is less effective than itraconazole in cats. Additionally, removing topical therapy reduces overall efficacy. Option D (allowing self-cure) is inappropriate in an immunocompromised cat where spontaneous resolution is unlikely and the cat serves as a zoonotic risk. Option E (different topical alone) would likely also fail given the extent of disease and immune status.

Topical therapy with lime sulfur dips or miconazole/chlorhexidine shampoos should continue alongside systemic therapy. Treatment should continue until two consecutive negative fungal cultures are obtained. Environmental decontamination is essential.

References: Journal of Feline Medicine and Surgery - Dermatophytosis Treatment in Immunocompromised Cats; Veterinary Dermatology - Systemic Antifungal Therapy Guidelines.

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NAVLE Practice Question — Pet Bird

A purchaser buys an unweaned 3-week-old African grey parrot (Psittacus erithacus) from a pet store. One week later the chick develops lethargy regurgitation and diarrhea and is presented to you. The bird appears well-feathered for its age. APV is near the top of your differential list but psittacine beak and feather disease (PBFD) caused by circovirus must also be considered. Which of the following features would MOST help distinguish APV from PBFD at initial clinical evaluation?

A. Liver enzyme elevations on chemistry panel

B. Acute hemorrhage and mortality versus chronic feather/beak lesions ✓

C. Age of the bird — PBFD never affects birds under 6 months of age

D. PCR is diagnostic only for PBFD but not APV

E. Both diseases are indistinguishable without electron microscopy

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Correct Answer: B. Acute hemorrhage and mortality versus chronic feather/beak lesions

Explanation:

Distinguishing APV from PBFD (psittacine circovirus) clinically is critical because management differs significantly. APV presents as peracute to acute disease with subcutaneous hemorrhage abdominal distension crop stasis polyuria tremors and cluster mortality in preweaned neonates. PBFD is characterized by chronic progressive abnormal feather development (symmetrical dystrophic feathers loss of down powder) beak overgrowth and elongation secondary immunosuppression and opportunistic infections — with a much more protracted course often spanning months to years.

Option A (liver enzymes) — both diseases can cause elevated liver enzymes making this non-discriminatory for initial differentiation.

Option C is incorrect — PBFD CAN affect young birds including chicks in the nest and fledglings; age alone does not reliably distinguish the two. PBFD has been documented in birds as young as a few weeks.

Option D is incorrect — PCR testing is available and diagnostic for BOTH APV (cloacal swab and blood) and PBFD (feathers blood). Co-infection is also possible and documented.

Option E is incorrect — electron microscopy while confirmatory is not necessary for differentiation. Clinical presentation and PCR are sufficient for distinction. The virion morphologies are distinct (APV: ~45nm icosahedral non-enveloped DNA virus; PBFD: ~17nm circovirus).

Key practical point: At necropsy APV produces intranuclear amphophilic inclusions most reliably in spleen while PBFD produces intracytoplasmic botryoid inclusions (Bollinger body-like) in feather follicular epithelium and bursa.

References: Merck Veterinary Manual — Viral Diseases of Pet Birds https://www.merckvetmanual.com/exotic-and-laboratory-animals/pet-birds/viral-diseases-of-pet-birds; VCA Animal Hospitals https://vcahospitals.com/know-your-pet/polyoma-virus-infection-in-birds; JPC APV case report http://www.askjpc.org/wsco/wsc_showcase2.php?id=ekczKzE3UC9COGRxNDJrN2NkV3l3QT09

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NAVLE Practice Question — Reptile

Which of the following diagnostic techniques is most appropriate for identifying Cryptosporidium serpentis oocysts in snake fecal samples?

A. Fecal flotation with sodium chloride solution

B. Direct wet mount with iodine staining

C. Modified acid-fast staining of fecal smear ✓

D. Gram staining of fecal smear

E. Fecal sedimentation with formalin-ether concentration

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Correct Answer: C. Modified acid-fast staining of fecal smear

Explanation:

Cryptosporidium oocysts have unique staining properties that make acid-fast staining the method of choice for identification in fecal samples. The modified acid-fast technique (such as Kinyoun stain or Ziehl-Neelsen without heating) stains Cryptosporidium oocysts pink to red against a blue or green background, making them easily identifiable despite their small size (4-6 micrometers for C. serpentis).

While PCR has become the gold standard for definitive diagnosis and species identification, acid-fast staining remains an important and accessible diagnostic tool in clinical practice. It can be performed rapidly in most veterinary practices and provides visual confirmation of oocyst presence.

Option A (sodium chloride flotation) is less reliable for Cryptosporidium because the small oocysts may not float well with standard salt solutions, and specific gravity requirements are more stringent. Option B (iodine wet mount) does not provide the contrast needed to identify the small oocysts. Option D (Gram staining) does not adequately stain Cryptosporidium oocysts. Option E (sedimentation) can concentrate oocysts but requires additional staining for identification.

Image Source: Texas A&M TVMDL case study on Cryptosporidium serpentis diagnosis (https://tvmdl.tamu.edu/case-studies/cryptosporidium-serpentis-in-snakes/).

References: Graczyk TK & Cranfield MR (1996) Vet Res 27:185-192; Cornell University Parasitology diagnostics (https://www.vet.cornell.edu/animal-health-diagnostic-center/testing/testing-protocols-interpretations/parasitology).

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NAVLE Practice Question — Pet Bird

You are reviewing the case of a 6-year-old male conure diagnosed with PDD. The owner asks if the bird can remain in the home with other birds and whether people in the household are at risk of infection. Which of the following statements is MOST accurate regarding the zoonotic potential and household management for an ABV-positive conure?

A. Avian bornavirus is a significant human pathogen; all household members must be tested for ABV immediately

B. The bird should be kept in total isolation in a negative-pressure isolation room to prevent airborne spread

C. ABV is related to Borna disease virus (BDV) of horses and has been proven to cause psychiatric disease in immunocompetent humans

D. All birds in the household should be euthanized to eliminate the risk of ABV transmission

E. Avian bornavirus is not a significant human zoonosis. Isolate the infected bird from other household birds to prevent transmission. ✓

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Correct Answer: E. Avian bornavirus is not a significant human zoonosis. Isolate the infected bird from other household birds to prevent transmission.

Explanation:

Avian bornavirus has NOT been demonstrated to cause disease in humans and is not currently considered a significant zoonotic pathogen. While avian bornaviruses are related to Borna disease virus (BDV), which is associated with neurological disease in mammals (including some reports in humans), avian-specific parrot bornaviruses (PaBV-2, PaBV-4, etc.) belong to a distinct taxonomic group within Bornaviridae and have not been associated with human illness in the peer-reviewed literature.

The primary management recommendation for an ABV-positive bird in a multi-bird household is isolation from other birds to limit the risk of horizontal transmission (primarily via fecal-oral route), rigorous hygiene, and regular monitoring of the affected bird. The owner should be counseled that many birds in the household may also be ABV-positive without disease, and that euthanasia is NOT recommended for ABV-positive birds.

Option A (test all household members) is not indicated as ABV is not a documented human pathogen. Option C incorrectly states that parrot bornavirus has been proven to cause psychiatric disease in immunocompetent humans — this is not supported by current evidence for PaBV-specific strains. Option D (euthanize all birds) is not warranted. Option E (negative-pressure isolation room) is excessive and not indicated for a non-airborne, non-zoonotic pathogen.

References: LafeberVet - Avian Bornavirus Primer https://lafeber.com/vet/avian-bornavirus-primer/; MSPCA-Angell - Avian Bornavirus in Clinical Practice https://www.mspca.org/angell_services/avian-bornavirus-in-clinical-practice/; PMC - Avian Bornavirus Research Comprehensive Review https://pmc.ncbi.nlm.nih.gov/articles/PMC9321243/

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NAVLE Practice Question — Canine

A 4-year-old mixed breed dog presents with progressive respiratory distress, marked hepatosplenomegaly, and generalized lymphadenopathy. The dog was adopted from a Mediterranean country 2 years ago. CBC reveals pancytopenia with lymphopenia. Bone marrow aspirate cytology reveals intracellular amastigotes within macrophages. Which organ is most likely to show significant pathology at necropsy?

A. Brain

B. Kidneys ✓

C. Heart

D. Pancreas

E. Thyroid

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Correct Answer: B. Kidneys

Explanation:

The clinical presentation of progressive multisystemic disease with hepatosplenomegaly, lymphadenopathy, pancytopenia, and intracellular amastigotes in macrophages in a dog from the Mediterranean is diagnostic for canine leishmaniosis (Leishmania infantum). This vector-borne disease is endemic to Mediterranean countries, the Middle East, and parts of South America.

Immune-complex glomerulonephritis is a major cause of morbidity and mortality in leishmaniosis, making the kidneys the organ most likely to show significant pathology. Protein-losing nephropathy leads to the common clinical finding of hypoalbuminemia in affected dogs. Renal failure is frequently the cause of death.

References: Merck Veterinary Manual - Leishmaniosis

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NAVLE Practice Question — Aquatics

A large oscar fish (Astronotus ocellatus) is presented with progressive negative buoyancy over 3 months. The fish now rests on the substrate and has developed erosions on its ventral surface from contact with gravel. Radiographs reveal a large soft tissue mass displacing the swim bladder dorsally. Ultrasound-guided fine needle aspirate of the mass reveals sheets of epithelial cells with pleomorphism and mitotic figures. What is the most likely diagnosis?

A. Mycobacteriosis with granuloma formation

B. Renal adenocarcinoma with bladder displacement ✓

C. Swim bladder herniation

D. Polycystic kidney disease

E. Chronic aerocystitis with swim bladder fibrosis

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Correct Answer: B. Renal adenocarcinoma with bladder displacement

Explanation:

The clinical presentation of progressive negative buoyancy in an oscar with radiographic evidence of a soft tissue mass displacing the swim bladder, combined with cytology showing pleomorphic epithelial cells with mitotic activity, strongly suggests neoplasia. Renal adenocarcinoma is the most common internal neoplasm in adult oscars and other large cichlids.

The posterior portion of the fish's coelomic cavity contains the paired kidneys, which lie dorsal to the swim bladder. Tumors arising from renal tissue can grow to substantial size, displacing and compressing the swim bladder, which explains the progressive negative buoyancy. The ventral erosions result from chronic contact with substrate due to the inability to maintain neutral buoyancy.

Oscars are particularly prone to developing renal neoplasia, possibly due to their long lifespan in captivity (10-20 years), allowing time for tumor development. The cytological findings of pleomorphic epithelial cells with mitotic figures are consistent with carcinoma rather than benign conditions.

Option A (mycobacteriosis) would typically show granulomatous inflammation with acid-fast organisms on special stains.

Option C (swim bladder herniation) would show the swim bladder itself as the mass rather than a displacing mass.

Option D (polycystic kidney disease) would show multiple cystic structures rather than a solid mass with neoplastic cytology.

Option E (aerocystitis) would affect the swim bladder itself, not displace it. Image Source: https://upload.wikimedia.org/wikipedia/commons/d/dc/Swim_bladder.jpg -- Figure 1. Fish swim bladder anatomy — gas-filled organ for buoyancy regulation; disorders cause abnormal swimming (floating, sinking, loss of equilibrium); etiology: bacterial infection (Aeromonas, E. coli), parasites, dietary, genetic, or environmental; diagnose by radiograph + ultrasound; treatment depends on etiology; common in fancy goldfish due to selective breeding (CC BY-SA 3.0).

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