**Core Concept**
Guanylyl cyclase is an enzyme involved in the production of cyclic guanosine monophosphate (cGMP), a key signaling molecule in various cellular processes. However, in the context of second messengers, cGMP is not considered a second messenger itself but rather a downstream effector molecule. Second messengers are molecules that transmit signals from the cell surface to the interior of the cell, often in response to hormone binding or neurotransmitter release.

**Why the Correct Answer is Right**
Guanylyl cyclase is an enzyme responsible for converting guanosine triphosphate (GTP) into cGMP. While cGMP is an important signaling molecule, it is not a second messenger in the classical sense. Instead, it acts as a downstream effector molecule that activates or inhibits various protein kinases, leading to changes in cellular function. In contrast, c-AMP, IP3, and DAG are all considered second messengers because they directly transmit signals within the cell in response to external stimuli.

**Why Each Wrong Option is Incorrect**
**Option A:** c-AMP is a second messenger that plays a crucial role in various cellular processes, including the regulation of gene expression and metabolism. It is produced by the activation of adenylate cyclase, which converts ATP into c-AMP.

**Option B:** IP3 (inositol trisphosphate) is a second messenger that activates calcium channels in the endoplasmic reticulum, leading to an increase in intracellular calcium concentrations. This, in turn, triggers various cellular responses, including muscle contraction and neurotransmitter release.

**Option D:** Diacylglycerol (DAG) is a second messenger that activates protein kinase C (PKC), a family of protein kinases involved in various cellular processes, including cell growth and differentiation.

**Clinical Pearl / High-Yield Fact**
The production of second messengers, such as c-AMP and IP3, is often regulated by G-protein coupled receptors (GPCRs), which are the largest family of membrane receptors in the human body. Understanding the role of second messengers and GPCRs is crucial in the diagnosis and treatment of various diseases, including cardiovascular disease and cancer.

**✓ Correct Answer: C. Guanylyl cyclase**