**Core Concept:** Angina pectoris is a clinical condition characterized by episodic chest pain due to reduced blood flow to the heart muscle, often triggered by increased demand or decreased supply of oxygen. Propanolol is a beta-blocker that can be used to manage angina by reducing the heart rate, decreasing oxygen demand, and blocking the stimulatory effects of catecholamines. However, beta-blockers can exacerbate angina in some patients, particularly those with a history of poor cardiac reserve or heart failure.

**Why the Correct Answer is Right:** In this case, propanolol is contraindicated in a patient already receiving a nitrate, as both medications have overlapping effects on the cardiovascular system. Nitrates dilate blood vessels, increase cardiac output, and reduce oxygen demand, while propanolol reduces heart rate and oxygen demand. Combining these two medications can lead to a significant reduction in cardiovascular reserve, potentially causing severe bradycardia, hypotension, and worsening of angina.

**Why Each Wrong Option is Incorrect:**

A. Contraindications in patients with angina include beta-blockers like propanolol. However, this option is incorrect because the patient is already receiving a nitrate, which makes the combination dangerous.

B. Beta-blockers are sometimes used in patients with angina, but this option is incorrect because the patient is receiving a nitrate, making the combination unsafe.

C. This option is incorrect because it mentions a different class of drugs (calcium channel blockers) that are not relevant to the question.

D. This option is incorrect because it describes a different condition (hypertension) unrelated to the patient's primary problem, angina.

**Clinical Pearl:** When treating patients with angina, it is crucial to carefully consider the medications the patient is already receiving and avoid combining medications with overlapping effects. In such cases, choosing an alternative therapy, such as calcium channel blockers, is recommended.