**Question:** A 17-year-old boy fell from his motorcycle and complains of numbness of lateral part of the arm. Examination reveals the axillary nerve is severed. Which of the following types of axons is most likely spared?

A. Sensory axons
B. Motor axons
C. Vagus axons
D. Postganglionic sympathetic axons

**Core Concept:** Axons are the long, slender projections of neurons that conduct electrical impulses. In the context of a severed axillary nerve, we need to consider the types of axons that are less likely to cause numbness in the lateral part of the arm due to axillary nerve injury.

**Why the Correct Answer is Right:** In the axillary nerve, sensory axons are responsible for transmitting sensory information (e.g., pain, temperature, and proprioception) from the arm to the central nervous system. The axillary nerve contains both sensory and motor axons, but in the context of a severed axillary nerve, the sensory axons are more likely to be responsible for the numbness in the lateral part of the arm. The correct answer is **A. Sensory axons**.

**Why Each Wrong Option is Incorrect:**

**Option B (Motor axons):** Severing the axillary nerve would primarily affect motor function, causing weakness or paralysis of the muscles innervated by the axillary nerve, but not numbness.

**Option C (Vagus axons):** The vagus nerve is responsible for parasympathetic nerves and regulates various functions such as heart rate, swallowing, and cough reflex. It does not have a direct connection with the lateral part of the arm, making it irrelevant in this context.

**Option D (Postganglionic sympathetic axons):** The postganglionic sympathetic axons are responsible for the sympathetic nervous system's effects on the arm, including vasoconstriction, increased heart rate, and sweating. A severed axillary nerve would not affect these functions, so they are not relevant in this context.

**Clinical Pearl:** In a case of axillary nerve injury, it is essential to differentiate between sensory and motor axons. A severed axillary nerve would primarily affect motor function, causing weakness or paralysis of the muscles innervated by the axillary nerve, but not numbness.

**Correct Answer Explanation:** In this scenario, the axillary nerve is severed, which primarily affects the motor function of the arm. Sensory axons, on the other hand, are responsible for transmitting sensory information (pain, temperature, and proprioception) from the arm to the central nervous system.

**Why the Correct Answer is Right:** The axillary nerve carries motor axons, which are responsible for muscle function and movement control in the arm. Severing the axillary nerve would primarily impair motor function, causing weakness or paralysis of the muscles innervated by the axillary nerve but not numbness. Sensory axons are responsible for transmitting sensory information (pain, temperature, and proprioception) from the arm to the central