Which of the following are NOT included in the four primary drug-receptor families?

A) Ligand-gated ion channels
B) G protein-coupled receptor systems
C) Antagonists
D) Transcription factors

C) Antagonists
Antagonists are NOT included in the four primary drug-receptor families. They are drugs or other agents that block or antagonize the effects of other substances or functions. The four primary drug-receptor families include: enzymes embedded in cell membranes, ligand-gated ion channels, G protein-coupled receptor systems, and transcription factors.