**Core Concept:** Proptosis refers to an abnormal protrusion of the eyeball, usually resulting from inflammation or mass effect on the orbit. Heat intolerance and palpitations suggest involvement of the autonomic nervous system. The sympathetic pathway controls heart rate and dilation of blood vessels, leading to increased blood flow in response to heat, which can result in palpitations and heat intolerance.

**Why the Correct Answer is Right:** The correct answer is **D. Hyperthyroidism** because it can cause proptosis due to mass effect on the orbit from the enlarged thyroid gland. Hyperthyroidism also leads to activation of the sympathetic nervous system, resulting in increased heart rate and blood flow, which explains the heat intolerance and palpitations.

**Why Each Wrong Option is Incorrect:**

A. **Goiter:** A goiter is an enlarged thyroid gland but typically does not cause proptosis, as the gland is usually located within the neck and does not exert significant mass effect on the orbit directly.

B. **Mass lesion:** While a mass lesion can cause proptosis, this option is too vague and does not specifically address the autonomic nervous system involvement.

C. **Orbital inflammation:** While orbital inflammation can lead to proptosis, it does not directly explain the autonomic nervous system involvement.

**Clinical Pearl:** A high index of suspicion is required in cases of proptosis, as it can be an early sign of hyperthyroidism and prompt intervention is crucial to prevent serious complications like optic neuropathy.

**Correct Answer:** .