**Core Concept**
The atrial muscle of the heart produces a hormone, known as atrial natriuretic peptide (ANP), in response to increased stretch and pressure in the atria. This hormone plays a crucial role in regulating blood pressure and electrolyte balance by promoting the excretion of sodium and water.

**Why the Correct Answer is Right**
Atrial natriuretic peptide (ANP) is secreted by the atrial myocytes in response to increased stretch and pressure, leading to the activation of guanylyl cyclase receptors. This activation results in the production of cyclic guanosine monophosphate (cGMP), which in turn activates protein kinase G (PKG). PKG phosphorylates and activates various downstream targets, including the sodium-potassium pump, leading to increased sodium excretion in the urine and subsequent vasodilation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the hormone produced by the atrial muscle of the heart. While the heart does produce various hormones, ANP is the primary hormone associated with atrial stretch and pressure.
**Option B:** This option is incorrect because it is not directly related to the hormone produced by the atrial muscle of the heart. While certain peptides are involved in cardiovascular regulation, ANP is the specific hormone produced in response to atrial stretch and pressure.
**Option C:** This option is incorrect because it is a hormone associated with the ventricles, not the atria. B-type natriuretic peptide (BNP) is secreted by the ventricular myocytes in response to increased stretch and pressure.

**Clinical Pearl / High-Yield Fact**
Atrial natriuretic peptide (ANP) is often elevated in patients with heart failure, and its levels can be used as a biomarker to assess disease severity and monitor treatment response.

**Correct Answer:** C. Atrial natriuretic peptide (ANP) is the hormone produced by the atrial muscle of the heart.