Tachyphylaxis is best described as:

• A. A slower onset of drug action with repeated dosing.
• B. A total loss of drug effect after a single dose.
• C. Rapid decrease in response to a drug after repeated dosing.
• D. A heightened response to a drug after repeated dosing.

Answer: (C)

Rapid loss of drug effect after repeated exposure is tachyphylaxis. With repeated dosing, the body's response diminishes quickly, even when the same dose is given. This happens because the system adapts—receptors may desensitize or signaling pathways may become less responsive, and sometimes mediators are depleted—so the drug’s effect weakens faster than expected.

Why the other patterns don’t fit: a slower onset implies a delay in when the drug begins to act rather than a reduced magnitude of effect; a total loss after a single dose is an immediate nonresponse, not a rapid decline with repeated use; and a heightened response after repeated dosing is sensitization, the opposite of tachyphylaxis.