Final answer:
The correct answer is 'Methacholine'. It is used in bronchial challenge tests because it can stimulate muscarinic receptors leading to bronchoconstriction, therefore revealing airway hyperreactivity, common in asthma patients. The correct option is B.
Step-by-step explanation:
The direct-acting cholinergic agent often used in bronchial challenge tests to assess the degree of airway activity is Methacholine. Methacholine acts by stimulating muscarinic receptors in the airway, leading to bronchoconstriction.
This makes it useful for assessing airway hyperreactivity in conditions like asthma. In contrast, agents such as edrophonium, neostigmine, and phospholine have different clinical uses and are not typically used for bronchial challenge tests.
Atropine is an anticholinergic drug which is standard for managing symptoms of nerve agent poisoning, as it blocks muscarinic receptors, preventing the overactivity caused by increased acetylcholine. Substances that inhibit acetylcholinesterase, such as organophosphates and certain medications, result in increased levels of acetylcholine, leading to prolonged muscle contraction and excessive parasympathetic actions.
Treatment for nerve agent poisoning includes anticholinergics to manage symptoms and oximes to regenerate acetylcholinesterase activity, which is necessary for breaking down acetylcholine.
Therefore, the correct option is B.