A 17-year-old girl is admitted to the hospital with severe dyspnea. Physical examination reveals that the patient is suffering from an asthma attack, with associated bronchospasm. Which of the following nerves is responsible for the innervation of the bronchial smooth muscle cells?

**Core Concept**
The **autonomic nervous system** plays a crucial role in regulating the tone of **bronchial smooth muscle cells**. The **parasympathetic nervous system** is responsible for promoting bronchospasm, while the **sympathetic nervous system** causes bronchodilation.

**Why the Correct Answer is Right**
The **vagus nerve**, which is the primary nerve responsible for parasympathetic innervation of the lungs, releases **acetylcholine** that binds to **muscarinic receptors** on bronchial smooth muscle cells, leading to bronchoconstriction.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not the primary nerve involved in bronchial smooth muscle innervation.
**Option B:** Incorrect as it is not directly related to the parasympathetic control of airway smooth muscle.
**Option D:** Incorrect because it is involved in the sympathetic nervous system, which generally causes bronchodilation.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **anticholinergic agents**, such as **ipratropium bromide**, can be used to treat bronchospasm by blocking the action of acetylcholine on muscarinic receptors.

**Correct Answer:** C. Vagus nerve.

✓ Correct Answer: C. Vagus