## **Core Concept**
The question tests understanding of muscarinic receptor subtypes, specifically M2 receptors, and their effects on the heart. Muscarinic receptors are part of the parasympathetic nervous system and are activated by acetylcholine. The M2 receptor subtype is predominantly found in the heart.

## **Why the Correct Answer is Right**
Activation of postsynaptic M2 receptors on the heart is associated with a decrease in heart rate (bradycardia) and a decrease in contractility. This is because M2 receptors are coupled to Gi/o proteins, which inhibit adenylate cyclase activity. This inhibition leads to a decrease in intracellular cyclic AMP (cAMP) levels. Since cAMP is involved in increasing heart rate and contractility (through protein kinase A activation), a decrease in cAMP results in decreased heart rate and contractility.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because activation of M2 receptors does not lead to an increase in heart rate; instead, it causes bradycardia.
- **Option B:** This option is incorrect because while M2 receptor activation does decrease contractility, the most directly associated and notable effect relevant to the question is the decrease in heart rate.
- **Option C:** This option might seem plausible because it could relate to effects on the heart, but it is not directly associated with M2 receptor activation effects listed.
- **Option D:** This option is incorrect because it does not accurately reflect the primary effect of M2 receptor activation on the heart.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that muscarinic M2 receptors in the heart are responsible for the parasympathetic control of heart rate, primarily through inducing bradycardia. This is a critical concept in pharmacology and physiology, especially relevant for exams like NEET PG, USMLE, AIIMS, and FMGE.

## **Correct Answer:** . Decreased heart rate.