**Question:** Increase in cAMP is caused by

A. activation of adenyl cyclase
B. inhibition of phosphodiesterase
C. stimulation of protein kinase A
D. stimulation of hormone receptors

**Core Concept:**

cAMP (cyclic adenosine monophosphate) is a second messenger molecule involved in various cellular processes, including gene expression, cell growth, and differentiation. It plays a crucial role in cellular responses to hormones and neurotransmitters. The primary way cAMP levels are regulated is through the actions of adenyl cyclase, phosphodiesterase, and protein kinase A.

**Why the Correct Answer is Right:**

Adenyl cyclase (AC) is a membrane-bound enzyme that catalyzes the conversion of ATP (adenosine triphosphate) to cAMP in the presence of hormones or neurotransmitters. The correct answer (A) is right because adenyl cyclase activation leads to increased production of cAMP upon stimulation of hormone receptors.

**Why Each Wrong Option is Incorrect:**

B. Inhibition of phosphodiesterase (PDE) would result in decreased degradation of cAMP, leading to decreased cAMP levels, not increased levels.
C. Stimulation of protein kinase A (PKA) would lead to decreased cAMP levels, as PKA phosphorylates and inhibits AC, decreasing cAMP production.
D. Stimulation of hormone receptors increases cAMP production, but option D is incorrect because it does not specifically address the mechanism by which cAMP levels are increased.

**Clinical Pearl:**

Understanding the regulation of cAMP levels is essential for grasping the molecular mechanisms of various physiological and pathological processes, including the actions of hormones, neurotransmitters, and drugs such as beta-adrenergic agonists. This knowledge is vital for interpreting laboratory test results (e.g., in thyroid hormone assays) and making informed clinical decisions regarding endocrinological disorders (e.g., in the management of hyperthyroidism).