Which of the following statements concerning the ELISA test for the presence of antibodies to the human immunodeficiency virus (HIV; the AIDS virus) is true?

Correct Answer: C

Rationale: In low-risk populations, the ELISA test’s positive predictive value is low due to a higher chance of false positives relative to true positives. (A) is false (most untreated HIV-positive individuals progress to AIDS), (B) is false (antibodies indicate infection, not immunity), (D) is false (false positives are rare in advanced AIDS), and (E) is false (negative ELISA doesn’t rule out early infection).

Correct Answer: B

Rationale: Hormone replacement therapy reduces osteoporosis risk, notably hip fractures, making (B) true. Combination therapy lowers endometrial cancer risk, breast cancer risk may increase, coronary benefits are debated, and thromboembolism risk rises slightly.

Correct Answer: D

Rationale: DDT was critical post-WWII in controlling typhus by killing lice, the vector, breaking transmission cycles.

Correct Answer: D

Rationale: Hepatitis A can be prevented with immunoglobulin (passive immunity) post-exposure, unlike the others, which rely on active vaccination.

Correct Answer: B

Rationale: Pertussis vaccine is associated with seizures (rarely), not arthus reactions, hyperglycemia, or hypothermia.