Which of the following is a second messenger?

• A. Acetylcholine
• B. Epidermal growth factor (EGF)
• C. Insulin
• D. cAMP

Answer: (D)

Verified step by step guidance

1. Understand the concept of a second messenger: A second messenger is a small intracellular signaling molecule that is produced or released in response to an extracellular signal (first messenger) and helps propagate the signal within the cell.
2. Review the examples of first messengers: Acetylcholine, Epidermal Growth Factor (EGF), and Insulin are all examples of first messengers. These are signaling molecules that bind to receptors on the cell surface to initiate a signaling cascade.
3. Identify the role of cAMP: Cyclic adenosine monophosphate (cAMP) is a well-known second messenger. It is synthesized from ATP by the enzyme adenylyl cyclase, which is activated by certain receptors (e.g., G-protein-coupled receptors).
4. Differentiate between first and second messengers: First messengers (e.g., acetylcholine, EGF, insulin) are extracellular molecules that bind to receptors, while second messengers (e.g., cAMP) are intracellular molecules that relay the signal inside the cell.
5. Conclude that cAMP is the correct answer: Based on the definitions and roles, cAMP is the second messenger in this context, while the other options are first messengers.