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#### usmle0987

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1. Eight hundred motor vehicle collisions occurred over 2 years in a city with a population of 600,000. 600 of these accidents were alcohol related. During the same period, there were 400 fatalities due to motoR vehicle collisions of which 75% were alcohol related. Which of the following best represents the 1 year incidents of alcohol related motor-vehicle fatalities in this community.
(A) 150 per 600,000
(B) 300 per 600,000
(C) 450 per 600,000
(D) 600 per 600,000
(E) Unable to determine from available information.

2. A 35 yearold woman at 18 weeks gestation has an increases serum AFP concentration. Which of the following has greatest influence in determining the predictive value of this tests in neural tube defects.
(A) Concentration of AFP in maternal serum.
(B) Maternal history ofmonozygotic twin pregnancies.
(C) Prevalence of neural tube defects in the population.
(D) Sensitivity of the test.
(E) Specificity of the test.

#### doctorsmonsters

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For question 1, I think its A
In 2 years its 300/600,000, so for 1 year it would 150/600,000. So A seems like a logical answer to me.

For 2, I think its B
Predictive values are related to the prevalence of a disease. The higher the prevalence, the higher is the positive predictive value and the lower the negative predictive value.

#### Mondoshawan

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I guess I would have gone for A and E.

A. 150/600,000 for the same reason as doctorsmonters.

E. Specificity, because it has the strongest effect on positive predictive value.
Maternal history and prevalence would affect pre-test probability, sensitivity would affect negative predictive value, and [afp] would inform a probability based on the test's predictive value but not the predictive value of the test itself.

#### Taiyakikung

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For q1 I think the best ans is A with the same reason as two rep above
For q2 I would go with C.Prevalence of neural tube defects in the population.
If the prevalence in the population increases the Positive predictive value will increase.

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