**Core Concept**
The patient's symptoms of bilateral proptosis, heat intolerance, and palpitations suggest a diagnosis related to the thyroid gland, specifically a condition that affects the orbit and cardiovascular system. Thyrotoxicosis is a common underlying condition in these patients.

**Why the Correct Answer is Right**
The most likely diagnosis in this scenario is Graves' disease, an autoimmune disorder that causes hyperthyroidism and leads to exophthalmos (bulging of the eyes) due to the deposition of mucopolysaccharides in the orbit. The increased metabolic rate in Graves' disease causes heat intolerance and palpitations. However, the most unlikely diagnosis here would be one that does not cause exophthalmos or is not primarily associated with thyroid dysfunction.

**Why Each Wrong Option is Incorrect**
**Option A:** Thyroiditis - This condition is characterized by inflammation of the thyroid gland, which can lead to hypothyroidism or hyperthyroidism. However, it is not typically associated with bilateral proptosis.

**Option B:** Medullary thyroid carcinoma - This is a type of thyroid cancer that arises from the parafollicular cells. While it can cause palpitations due to the secretion of calcitonin, it is not typically associated with bilateral proptosis or heat intolerance.

**Option C:** De Quervain thyroiditis - This is a form of thyroiditis that is characterized by inflammation of the thyroid gland, leading to pain and tenderness. While it can cause heat intolerance, it is not typically associated with bilateral proptosis.

**Clinical Pearl / High-Yield Fact**
Graves' disease is an autoimmune disorder that affects approximately 1 in 100 people, with a higher incidence in women. It is essential to consider the orbit and cardiovascular systems when evaluating patients with thyroid dysfunction.

**Correct Answer: D.**