**Core Concept**
Non-selective beta blockers can cause bronchoconstriction in asthmatic patients by blocking beta-2 receptors in the lungs, leading to a severe exacerbation of asthma symptoms.

**Why the Correct Answer is Right**
Non-selective beta blockers, such as propranolol, inhibit both beta-1 and beta-2 adrenergic receptors. Beta-2 receptors are responsible for bronchodilation, and blocking these receptors can lead to bronchoconstriction, making non-selective beta blockers contraindicated in asthma patients. This is particularly concerning in patients with pre-existing asthma, as it can trigger a severe asthma attack.

**Why Each Wrong Option is Incorrect**
**Option A:** Selective beta-1 blockers, such as metoprolol, are not contraindicated in asthma patients because they preferentially block beta-1 receptors, sparing beta-2 receptors and avoiding bronchoconstriction.
**Option B:** Alpha blockers, such as phentolamine, do not affect beta receptors and are not contraindicated in asthma patients.
**Option C:** Calcium channel blockers, such as verapamil, do not affect beta receptors and are not contraindicated in asthma patients.

**Clinical Pearl / High-Yield Fact**
When prescribing beta blockers, it is essential to consider the patient's medical history, including any pre-existing respiratory conditions like asthma. Selective beta-1 blockers are generally safer in these patients, but it is crucial to monitor for any signs of bronchoconstriction or asthma exacerbation.

**Correct Answer: D. Non-selective beta blockers.**