Which β-blocker can exacerbate a patient's asthma symptoms by causing bronchoconstriction?

Propranolol is a non-selective β-blocker, which means it blocks both β1 and β2 adrenergic receptors. While β1 receptors are primarily found in the heart and their blockade is useful for managing cardiovascular conditions, β2 receptors are located in airway smooth muscle and their blockade can lead to bronchoconstriction. This can exacerbate symptoms in patients with asthma or reactive airway disease.

In contrast, the other options provided are selective β1-blockers or have a different mechanism of action. Metoprolol and Atenolol are selective β1-blockers, which primarily affect the heart and have minimal impact on bronchial smooth muscle, making them safer choices for patients with asthma. Labetalol, while it has both α and β-blocking properties, is often more appropriately used in the management of hypertension and does not typically exacerbate asthma as significantly as non-selective agents like Propranolol. Therefore, Propranolol's non-selective action clearly explains why it can worsen asthma symptoms.