**Question:** Which of the following is a membrane receptor defect -

A. Ligand-gated ion channels
B. G-protein coupled receptors
C. Receptor tyrosine kinases
D. Cyclooxygenase enzymes

**Core Concept:** Membrane receptors are proteins embedded in the cell membrane, which play a crucial role in mediating cellular responses to various stimuli, including hormones, neurotransmitters, and drugs. Membrane receptors can be broadly classified into four categories based on their mode of action:

1. Ligand-gated ion channels: These receptors open or close ion channels in response to specific ligands (hormones or neurotransmitters). They are involved in transmitting signals across the cell membrane, such as neurotransmitters in the central nervous system.
2. G-protein coupled receptors (GPCRs): These receptors are activated by small molecules (ligands) and are involved in various physiological processes, including regulation of cell growth, differentiation, and secretion of hormones and neurotransmitters. GPCRs are also involved in drug action, including the mode of action of many medications.
3. Receptor tyrosine kinases: These receptors are involved in cell growth, differentiation, and signal transduction. They are activated by growth factors and play a significant role in cancer biology, as well as in drug action, particularly those targeting cancer cells.
4. Cyclooxygenase enzymes (COX): These enzymes are involved in the synthesis of prostaglandins and thromboxanes, which are involved in inflammation, pain, fever, and blood clotting. They are not membrane receptors, but rather enzymes involved in prostaglandin synthesis.

**Why the Correct Answer is Right:**

B. G-protein coupled receptors (GPCRs) are membrane receptors that are involved in various physiological processes and drug action. They are activated by small molecules, and upon activation, they signal through G proteins, which in turn activate intracellular enzymes, ion channels, or other signaling molecules. GPCRs are crucial for understanding drug action as they are the target of many medications.

**Why Each Wrong Answer is Incorrect:**

A. Ligand-gated ion channels are involved in transmitting signals across the cell membrane, particularly neurotransmitters in the central nervous system. They are not involved in drug action as they are not the target of many medications.

C. Receptor tyrosine kinases are involved in cell growth, differentiation, and signal transduction, particularly in cancer biology. They are involved in drug action, particularly in targeting cancer cells.

D. Cyclooxygenase enzymes (COX) are involved in prostaglandin synthesis, which is involved in inflammation, pain, fever, and blood clotting. They are not membrane receptors but rather enzymes involved in prostaglandin synthesis.