What is a second messenger?

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A second messenger is a cytoplasmic signaling molecule produced after a ligand binds to a cell-surface receptor, and it carries and amplifies the signal inside the cell to trigger downstream effects. The ligand is the first messenger, and the molecule generated inside the cytoplasm in response to receptor activation is the second messenger. Examples include cyclic AMP, calcium ions, IP3, and DAG, which activate kinases or open ion channels to propagate the response.

This explains why the correct choice is a cytoplasmic molecule produced as a result of receptor-ligand interaction that mediates the response. A lipid in the membrane is a structural component, not a signal generated in response to receptor activation. A transcription factor is a downstream effector involved in changing gene expression, not the immediate messenger that relays the signal inside the cell. A membrane receptor is the site of signal detection, not the intracellular messenger that transmits the signal.