An example of a drug that may receive expedited medical approval would be:

a. A drug used to treat a rare disease that affects less than 200,000 people
b. A placebo that is administered to provide psychological control of responses
c. A drug that may help prevent a public health threat such as HIV
d. A drug that is being evaluated in order to determine the therapeutic index

c
Rationale: a. This answer describes the definition of an orphan drug.
b. A placebo is used in testing and does not require expedited medical approval.
c. Expedited approval may be granted when a drug may be of use in treating a public health threat such as HIV.
d. The therapeutic index is obtained during preclinical phase of drug testing and does not require expedited approval.