**Core Concept**
cGMP (cyclic guanosine monophosphate) is a crucial second messenger molecule involved in cellular signaling pathways, particularly in the cardiovascular system and smooth muscle relaxation. It plays a key role in mediating the effects of various hormones and neurotransmitters.

**Why the Correct Answer is Right**
cGMP acts as a second messenger for both atrial natriuretic factor (ANF) and nitric oxide (NO), also referred to as nitric acid in this context. ANF, released by the atria in response to increased stretch and pressure, binds to its receptor, leading to activation of guanylyl cyclase and subsequent production of cGMP. This increase in cGMP levels causes smooth muscle relaxation in blood vessels, resulting in vasodilation and decreased blood pressure. Similarly, nitric oxide, produced in endothelial cells, diffuses into smooth muscle cells, where it activates guanylyl cyclase and increases cGMP levels, promoting vasodilation and relaxation.

**Why Each Wrong Option is Incorrect**
**Option A:** Atrial natriuretic factor does indeed use cGMP as its second messenger, so this option is incorrect because it's incomplete.
**Option B:** Nitric acid is a misnomer for nitric oxide; however, nitric oxide is the correct molecule, and it does use cGMP as its second messenger, making this option also incorrect because it's incomplete.
**Option D:** Both of the above hormones use cGMP as their second messenger, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
cGMP is a key second messenger in cardiovascular regulation, and its dysregulation is implicated in various cardiovascular diseases, including hypertension and heart failure. Understanding the role of cGMP in ANF and NO signaling is essential for grasping the pathophysiology of these conditions.

**✓ Correct Answer: C. Both of the above**