**Core Concept**
Chronotropism refers to the regulation of heart rate, either an increase (positive chronotropism) or decrease (negative chronotropism). This concept is essential in pharmacology, as various drugs can influence heart rate by acting on the autonomic nervous system or directly on the cardiac tissue.

**Why the Correct Answer is Right**
The correct answer, B. Drugs affecting heart rate, is a direct reflection of the definition of chronotropism. Chronotropes are drugs that alter the heart rate, either by stimulating the sympathetic nervous system (e.g., epinephrine, isoproterenol) or inhibiting the parasympathetic nervous system (e.g., atropine). These drugs can be used therapeutically to increase or decrease heart rate, depending on the clinical context. For example, isoproterenol is used to increase heart rate in patients with bradycardia or heart block.

**Why Each Wrong Option is Incorrect**
**Option A:** Drugs affecting the contractility of heart are referred to as inotropes, not chronotropes. Inotropes, such as dobutamine or dopamine, increase or decrease the contractility of the heart muscle, but do not directly affect heart rate.

**Option C:** Drugs affecting myocardial blood flow are referred to as vasodilators or vasoconstrictors, depending on their mechanism of action. These drugs can influence coronary blood flow, but do not directly affect heart rate.

**Option D:** Drugs affecting diastolic relaxation are referred to as lusitropes, not chronotropes. Lusitropes, such as calcium channel blockers, affect the relaxation of the heart muscle during diastole, but do not directly affect heart rate.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that chronotropes can have significant effects on cardiac output and blood pressure, depending on the direction and magnitude of the change in heart rate. Therefore, it is crucial to carefully evaluate the potential benefits and risks of chronotropic agents in clinical practice.

**✓ Correct Answer: B. Drugs affecting heart rate**