**Core Concept**
The clinical presentation suggests a systemic disorder affecting multiple organ systems, including the gastrointestinal, nervous, and endocrine systems. The combination of chronic constipation, headache, palpitations, marfanoid habitus, and specific physical findings points towards a **neuroendocrine disorder**. The presence of a thyroid nodule and other characteristic features hints at a **multiple endocrine neoplasia (MEN) syndrome**.

**Why the Correct Answer is Right**
Given the constellation of symptoms and signs, including neuromas of the tongue, medullated corneal nerve fibers, and a thyroid nodule, the diagnosis is likely **MEN 2B**, a subtype of multiple endocrine neoplasia. This condition is characterized by the presence of medullary thyroid carcinoma, pheochromocytoma, and other features such as marfanoid habitus and mucosal neuromas. The **RET proto-oncogene** mutation is commonly associated with MEN 2B.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice would not account for the full spectrum of symptoms and signs, particularly the neuromas and the specific type of thyroid nodule.
**Option B:** While this could explain some of the endocrine findings, it does not typically include neuromas or the specific constellation of features seen in this patient.
**Option C:** Similar to Option B, it might explain some endocrine aspects but not the full range of symptoms and signs, especially the neuromas and marfanoid habitus.

**Clinical Pearl / High-Yield Fact**
MEN 2B is distinguished from other MEN syndromes by the presence of mucosal neuromas, a marfanoid body habitus without ectopia lentis, and a high incidence of medullary thyroid carcinoma and pheochromocytoma. Early recognition is crucial due to the aggressive nature of the medullary thyroid carcinoma associated with this syndrome.

**Correct Answer:** D. Multiple Endocrine Neoplasia (MEN) type 2B.