NAVLE EVALUATION TEST QUESTIONS AND
ANSWERS SET A+
✔✔For the last 10 years, your state has had a mandatory vaccination program against
"pedunculated giblet disease" in fur-bearing turtles and the prevalence of this terrible
disease has decreased markedly. How does this decrease in prevalence affect the
predictive value positive (PVP) of the best serologic test for pedunculated giblet
disease?
A - PVP depends on the number tested, not prevalence
B - PVP is affected by specificity, not prevalence
C - PVP increases as prevalence decreases
D - PVP decreases as prevalence decreases
E - PVP stays the same as prevalence decreases - ✔✔D
Don't think too hard on this: As prevalence of a disease goes DOWN, PVP of your
serologic test also goes DOWN. That is, as your disease becomes more and more rare,
the predictive value of your same old test gets WORSE. That is all you need to know.
Read more only if you want to see the math, (but you don't need it.)
Lets say prevalence of pedunculated giblet disease is 30% in 1000 turtles (that's 300
infected, 700 disease-free, then). A test with 90% sensitivity would correctly Dx 270 (cell
"a") with the disease (true pos) and Correctly say 30 were negative (cell "c"); a test with
90% specificity would correctly Dx 630 (cell "d") as disease-free (true negs) and
Correctly say 70 were positive (cell "b"): PVP=a/(a+b)=270/(270+70)= 79%.
Now, do the same math on 1000 turtles, but assume prevalence is now only 1% (0.01)
after your vaccination program. You will see PVP goes down to 47%.
✔✔In September, a 5-year-old Quarterhorse mare is presented with a 5-day history of
left head tilt, facial paralysis, depression, and stumbling. The horse is ataxic and
,knuckles on both forelegs, worse on the left. There is muscle atrophy of the left
pectorals and right hindquarters and strips of localized spontaneous sweating over the
left trunk. The horse is immunoblot test-positive for Sarcocystis neurona. What
prognosis is given to the owner if she elects to treat the horse?
A - Only 25% of treated horses return to normal function
B - 50% of treated horses recover completely
C - Prognosis is good if treated with doxycycline
D - At least 60% of treated horses improve
E - Prognosis is grave - ✔✔D
The prognosis is guarded to fair for treatment of equine protozoal myeloencephalitis
(EPM). Less than 25% recover completely, but at least 60% of treated horses improve.
Horses that remain positive by immunoblot testing commonly relapse, but horses who
become negative by immunoblot testing rarely relapse.
✔✔A farmer comes to you with a new test for detection of mastitis in his heifers. He
claims that both the sensitivity and specificity of his test is 90%. Mastitis has a
prevalence of 5% in his herd. The negative predictive value (NPV) is approximately
_________.
A - 35%
B - 99%
C - 3%
D - 72% - ✔✔B
The correct answer is approximately 99%. The negative predictive value is the
proportion of test-negative animals that are truly disease negative (unaffected) in this
population. In the 2X2 table, where a= Disease +, Test +; b= Disease -, Test +; c=
Disease +, Test -; d= Disease -, Test -. The NPV is d/(c+d), or disease -, test - animals
DIVIDED by all of the test - animals.
The answer to the question has to consider the prevalence of mastitis in the herd, which
here is 5%. As the prevalence declines, NPV will increase.
The negative predictive value gives a proportion of the negative tests that are truly
negative. The value is the number of true negatives divided by the sum of the true
negatives and false negatives.
In this herd, the proportion of test negative animals that actually is negative is specificity
x (1-prevalence) or (0.9)x(0.95) = 0.855 (d). The proportion of false negatives in the
herd is (1-sensitivity) x prevalence, or (0.1)(0.05) = 0.005 (c).
Negative predictive value = d/(c+d) = 0.855/(0.855+0.005) = 0.994.
,✔✔A stray dog is presented after being hit by a car. The dog has hypotonic forelimbs
and spastic paresis in the hinds. All four limbs have proprioceptive deficits and
sensation loss-signs are worse in the forelimbs. Where is the lesion?
A - Cranial cervical: C1-C5
B - Cannot say without cutaneous trunci reflex results
C - Lumbosacral L4-S3
D - Thoracolumbar T3-L3
E - Cervicothoracic: C6-T2 - ✔✔E
Cervicothoracic: C6-T2. Weak, hypotonic (Lower motor neuron-LMN) forelimbs and
spastic paresis (Upper motor neuron-UMN) hindlimbs are signs of a cervicothoracic
(C6-T2) lesion. May see worse signs in fores then hinds.
NOTE: This is the OPPOSITE presentation as Schiff-Sherrington syndrome ie:
SEVERE acute spinal cord trauma T3-L3, with thoracic limb extensor rigidity (UMN) and
hind limb flaccid paralysis (LMN). Lesion is caudal to T2, typically see animal soon
(hours) after a bad trauma, like hit by car. With a typical, less severe T3-L3 lesion (like a
disc, perhaps), expect to see UMN hind limb signs and normal forelimbs.
Can localize T3-L3 lesion by checking cutaneous trunci reflex -The lesion is usually 1-2
vertebrae cranial to the line of analgesia (where dog does not feel pinching skin).
With C1-C5 would expect UMN signs in all 4 limbs, usually worse in hinds. With a T3-
L3, would see UMN hind limb signs and normal forelimbs.
✔✔What distinguishes lymphoma from leukemia?
A - Lymphoma is benign
B - Leukemia is derived only from myeloid cells
C - Leukemia is the precursor to lymphoma
D - Lymphoma originates in solid tissues
E - Lymphoma occurs only in dogs - ✔✔D
Lymphoma=solid organs and bone marrow. Leukemia=blood circulation and bone
marrow.
Lymphoma originates in solid tissues. In general, lymphoma and leukemia both arise
from malignant transformation of lymphoid cells and are named based on tumor
location. Lymphoma is a tumor of lymphocytes that originates mainly in solid lymphoid
tissues including the bone marrow, thymus, lymph nodes, and spleen. In addition,
common extranodal sites include the skin, eye, CNS, testis, and bone. Lymphomas are
further distinguished according to their location (multicentric, cutaneous, CNS, GI).
Leukemias can also arise from myeloid cells (monocytes, basophils, eosinoiphils,
megakaryocytes, erythrocytic precursor cells). Leukemia is a malignant neoplastic
, disease of the WBC or RBC precursors with neoplastic cells in the bone marrow and
peripheral blood. Leukemias are further distinguished according to their behaviour
(acute or chronic) and their precursor cell (lymphoid or myeloid), so you will see names
like Acute Lymphocytic Leukemia (ALL) or Chronic Myeloid Leukemia (CML).
In cats, FeLV infection is an important risk factor for both leukemia and lymphoma. FIV
infected cats have a higher than expected incidence of FeLV-negative lymphomas.
✔✔You have diagnosed nitrate poisoning in a group of yearling cattle which were
grazing Sudan grass. Which of the following is the most effective treatment?
A - Digitalis
B - Sodium thiosulfate
C - Vitamin B12
D - Methylene blue
E - Calcium gluconate - ✔✔D
Nitate toxicosis causes methemoglobinemia, in which the blood appears chocolate
brown. As a result the oxygen carrying capacity and delivery is greatly reduced and the
animal may die. Methylene blue is usually administered IV as a 1% to 4% solution, with
a total dose of 4 to 15 mg.Kg body weight to cattle, for treating nitrate toxicosis.
Nitrates are found in high levels in many plants and some water sources. The nitrate is
converted in the rumen to nitrite, absorbed into blood, which causes oxidation of the
heme iron to the +3 ferrous state (Called methemoglobin). Methylene blue restores the
iron in hemoglobin to its normal (reduced) oxygen-carrying state. This is achieved by
providing an artificial electron acceptor for NADPH methemoglobin reductase (RBCs
usually don't have one; the presence of methylene blue allows the enzyme to function at
5A? normal levels). The NADPH is generated via the hexose monophosphate shunt.
✔✔The Animal Welfare Act is enforced by which U.S. government agency?
A - Institutional Animal Care and Use Committee (IACUC)
B - Department of Agriculture (USDA)
C - Food and Drug Administration (FDA)
D - National Institute of Health (NIH)
E - Centers for Disease Control and Prevention (CDC) - ✔✔B
The United States Department of Agriculture (USDA) enforces the Animal Welfare Act.
Specifically, it is the Animal and Plant Health Inspection Service (APHIS) within the
USDA that is responsible.
The Animal Welfare act sets minimal standards for care of laboratory animals including
dogs, cats and non-human primates. It regulates the pet trade, animal transportation
and exhibition and licensure of animal dealers. Since 1976 it has prohibited most forms
of commercial animal fighting.
ANSWERS SET A+
✔✔For the last 10 years, your state has had a mandatory vaccination program against
"pedunculated giblet disease" in fur-bearing turtles and the prevalence of this terrible
disease has decreased markedly. How does this decrease in prevalence affect the
predictive value positive (PVP) of the best serologic test for pedunculated giblet
disease?
A - PVP depends on the number tested, not prevalence
B - PVP is affected by specificity, not prevalence
C - PVP increases as prevalence decreases
D - PVP decreases as prevalence decreases
E - PVP stays the same as prevalence decreases - ✔✔D
Don't think too hard on this: As prevalence of a disease goes DOWN, PVP of your
serologic test also goes DOWN. That is, as your disease becomes more and more rare,
the predictive value of your same old test gets WORSE. That is all you need to know.
Read more only if you want to see the math, (but you don't need it.)
Lets say prevalence of pedunculated giblet disease is 30% in 1000 turtles (that's 300
infected, 700 disease-free, then). A test with 90% sensitivity would correctly Dx 270 (cell
"a") with the disease (true pos) and Correctly say 30 were negative (cell "c"); a test with
90% specificity would correctly Dx 630 (cell "d") as disease-free (true negs) and
Correctly say 70 were positive (cell "b"): PVP=a/(a+b)=270/(270+70)= 79%.
Now, do the same math on 1000 turtles, but assume prevalence is now only 1% (0.01)
after your vaccination program. You will see PVP goes down to 47%.
✔✔In September, a 5-year-old Quarterhorse mare is presented with a 5-day history of
left head tilt, facial paralysis, depression, and stumbling. The horse is ataxic and
,knuckles on both forelegs, worse on the left. There is muscle atrophy of the left
pectorals and right hindquarters and strips of localized spontaneous sweating over the
left trunk. The horse is immunoblot test-positive for Sarcocystis neurona. What
prognosis is given to the owner if she elects to treat the horse?
A - Only 25% of treated horses return to normal function
B - 50% of treated horses recover completely
C - Prognosis is good if treated with doxycycline
D - At least 60% of treated horses improve
E - Prognosis is grave - ✔✔D
The prognosis is guarded to fair for treatment of equine protozoal myeloencephalitis
(EPM). Less than 25% recover completely, but at least 60% of treated horses improve.
Horses that remain positive by immunoblot testing commonly relapse, but horses who
become negative by immunoblot testing rarely relapse.
✔✔A farmer comes to you with a new test for detection of mastitis in his heifers. He
claims that both the sensitivity and specificity of his test is 90%. Mastitis has a
prevalence of 5% in his herd. The negative predictive value (NPV) is approximately
_________.
A - 35%
B - 99%
C - 3%
D - 72% - ✔✔B
The correct answer is approximately 99%. The negative predictive value is the
proportion of test-negative animals that are truly disease negative (unaffected) in this
population. In the 2X2 table, where a= Disease +, Test +; b= Disease -, Test +; c=
Disease +, Test -; d= Disease -, Test -. The NPV is d/(c+d), or disease -, test - animals
DIVIDED by all of the test - animals.
The answer to the question has to consider the prevalence of mastitis in the herd, which
here is 5%. As the prevalence declines, NPV will increase.
The negative predictive value gives a proportion of the negative tests that are truly
negative. The value is the number of true negatives divided by the sum of the true
negatives and false negatives.
In this herd, the proportion of test negative animals that actually is negative is specificity
x (1-prevalence) or (0.9)x(0.95) = 0.855 (d). The proportion of false negatives in the
herd is (1-sensitivity) x prevalence, or (0.1)(0.05) = 0.005 (c).
Negative predictive value = d/(c+d) = 0.855/(0.855+0.005) = 0.994.
,✔✔A stray dog is presented after being hit by a car. The dog has hypotonic forelimbs
and spastic paresis in the hinds. All four limbs have proprioceptive deficits and
sensation loss-signs are worse in the forelimbs. Where is the lesion?
A - Cranial cervical: C1-C5
B - Cannot say without cutaneous trunci reflex results
C - Lumbosacral L4-S3
D - Thoracolumbar T3-L3
E - Cervicothoracic: C6-T2 - ✔✔E
Cervicothoracic: C6-T2. Weak, hypotonic (Lower motor neuron-LMN) forelimbs and
spastic paresis (Upper motor neuron-UMN) hindlimbs are signs of a cervicothoracic
(C6-T2) lesion. May see worse signs in fores then hinds.
NOTE: This is the OPPOSITE presentation as Schiff-Sherrington syndrome ie:
SEVERE acute spinal cord trauma T3-L3, with thoracic limb extensor rigidity (UMN) and
hind limb flaccid paralysis (LMN). Lesion is caudal to T2, typically see animal soon
(hours) after a bad trauma, like hit by car. With a typical, less severe T3-L3 lesion (like a
disc, perhaps), expect to see UMN hind limb signs and normal forelimbs.
Can localize T3-L3 lesion by checking cutaneous trunci reflex -The lesion is usually 1-2
vertebrae cranial to the line of analgesia (where dog does not feel pinching skin).
With C1-C5 would expect UMN signs in all 4 limbs, usually worse in hinds. With a T3-
L3, would see UMN hind limb signs and normal forelimbs.
✔✔What distinguishes lymphoma from leukemia?
A - Lymphoma is benign
B - Leukemia is derived only from myeloid cells
C - Leukemia is the precursor to lymphoma
D - Lymphoma originates in solid tissues
E - Lymphoma occurs only in dogs - ✔✔D
Lymphoma=solid organs and bone marrow. Leukemia=blood circulation and bone
marrow.
Lymphoma originates in solid tissues. In general, lymphoma and leukemia both arise
from malignant transformation of lymphoid cells and are named based on tumor
location. Lymphoma is a tumor of lymphocytes that originates mainly in solid lymphoid
tissues including the bone marrow, thymus, lymph nodes, and spleen. In addition,
common extranodal sites include the skin, eye, CNS, testis, and bone. Lymphomas are
further distinguished according to their location (multicentric, cutaneous, CNS, GI).
Leukemias can also arise from myeloid cells (monocytes, basophils, eosinoiphils,
megakaryocytes, erythrocytic precursor cells). Leukemia is a malignant neoplastic
, disease of the WBC or RBC precursors with neoplastic cells in the bone marrow and
peripheral blood. Leukemias are further distinguished according to their behaviour
(acute or chronic) and their precursor cell (lymphoid or myeloid), so you will see names
like Acute Lymphocytic Leukemia (ALL) or Chronic Myeloid Leukemia (CML).
In cats, FeLV infection is an important risk factor for both leukemia and lymphoma. FIV
infected cats have a higher than expected incidence of FeLV-negative lymphomas.
✔✔You have diagnosed nitrate poisoning in a group of yearling cattle which were
grazing Sudan grass. Which of the following is the most effective treatment?
A - Digitalis
B - Sodium thiosulfate
C - Vitamin B12
D - Methylene blue
E - Calcium gluconate - ✔✔D
Nitate toxicosis causes methemoglobinemia, in which the blood appears chocolate
brown. As a result the oxygen carrying capacity and delivery is greatly reduced and the
animal may die. Methylene blue is usually administered IV as a 1% to 4% solution, with
a total dose of 4 to 15 mg.Kg body weight to cattle, for treating nitrate toxicosis.
Nitrates are found in high levels in many plants and some water sources. The nitrate is
converted in the rumen to nitrite, absorbed into blood, which causes oxidation of the
heme iron to the +3 ferrous state (Called methemoglobin). Methylene blue restores the
iron in hemoglobin to its normal (reduced) oxygen-carrying state. This is achieved by
providing an artificial electron acceptor for NADPH methemoglobin reductase (RBCs
usually don't have one; the presence of methylene blue allows the enzyme to function at
5A? normal levels). The NADPH is generated via the hexose monophosphate shunt.
✔✔The Animal Welfare Act is enforced by which U.S. government agency?
A - Institutional Animal Care and Use Committee (IACUC)
B - Department of Agriculture (USDA)
C - Food and Drug Administration (FDA)
D - National Institute of Health (NIH)
E - Centers for Disease Control and Prevention (CDC) - ✔✔B
The United States Department of Agriculture (USDA) enforces the Animal Welfare Act.
Specifically, it is the Animal and Plant Health Inspection Service (APHIS) within the
USDA that is responsible.
The Animal Welfare act sets minimal standards for care of laboratory animals including
dogs, cats and non-human primates. It regulates the pet trade, animal transportation
and exhibition and licensure of animal dealers. Since 1976 it has prohibited most forms
of commercial animal fighting.
