Which of the following is NOT a major receptor family described?

• A. Ion channels
• B. G-Protein coupled receptors
• C. Receptor tyrosine kinases
• D. Ligand-gated channels

Answer: (D)

Major receptor families described include ion channel receptors, G-protein-coupled receptors, receptor tyrosine kinases, and nuclear receptors. Ligand-gated channels are a type of ion channel receptor—opening a pore in response to a ligand to allow ions to flow, producing rapid changes in membrane potential. Because they’re part of the broader ion channel family rather than a separate major receptor family, they aren’t described as a distinct major family. The other options—ion channels, GPCRs, and receptor tyrosine kinases—are each recognized as independent major receptor families. So this item identifies ligandid-gated channels as not a separate major receptor family.