**Question:** True about G protein-coupled receptors is

A. Receptors that activate adenylyl cyclase and increase cAMP levels
B. Receptors that activate phospholipase C and increase inositol trisphosphate and diacylglycerol levels
C. Receptors that activate protein kinase C and increase calcium levels
D. Receptors that activate both adenylyl cyclase and protein kinase C pathways

**Correct Answer:** D. G protein-coupled receptors (GPCRs) can activate both adenylyl cyclase and protein kinase C pathways.

**Core Concept:** G protein-coupled receptors (GPCRs) are a large family of cell surface receptors involved in transmitting signals across the cell membrane. They are activated by a wide range of endogenous and exogenous molecules, including neurotransmitters, hormones, and odorants. GPCRs are crucial in regulating various physiological processes and signaling pathways.

**Why the Correct Answer is Right:** GPCRs are known to couple with different G proteins, which in turn activate various downstream signaling pathways. In the case of GPCRs that activate adenylyl cyclase, they typically couple with Gs proteins, leading to an increase in cAMP levels and activation of protein kinase A (PKA). On the other hand, GPCRs that activate phospholipase C (PLC) typically couple with Gq/11 proteins, resulting in the production of inositol trisphosphate (IP3) and diacylglycerol (DAG), which leads to the activation of protein kinase C (PKC).

**Why Each Wrong Option is Incorrect:**

A. The statement about adenylyl cyclase activation is true, but it does not cover that GPCRs can also activate other pathways like protein kinase C.
B. The statement about phospholipase C activation is correct, but it does not mention that GPCRs can activate other pathways like protein kinase A.
C. Activation of protein kinase C is the only pathway mentioned, which is incorrect as GPCRs can activate multiple pathways.
D. This option covers the activation of both adenylyl cyclase and protein kinase C pathways, which is the correct answer.

**Core Concept:** G protein-coupled receptors (GPCRs) are a large family of cell surface receptors involved in transmitting signals across the cell membrane. They are activated by a wide range of endogenous and exogenous molecules, including neurotransmitters, hormones, and odorants. GPCRs play a crucial role in regulating various physiological processes and signaling pathways.

**Why Each Wrong Option is Incorrect:**

A. Activation of adenylyl cyclase is true but does not account for GPCR activation of other pathways like protein kinase C.
B. Activation of phospholipase C is correct but does not account for GPCR activation of other pathways like protein kinase A.
C. Activation of protein kinase C is the only pathway mentioned, which is incorrect, as GPCRs can activate multiple pathways.
D. This option covers the activation of both adenylyl cyclase and protein kinase C pathways, which is the correct answer.

**Why the