Which condition includes features such as marfanoid habitus and medullary thyroid carcinoma?

The condition characterized by features such as marfanoid habitus and medullary thyroid carcinoma is MEN2B (Multiple Endocrine Neoplasia type 2B). This syndrome is a genetic condition caused by specific mutations in the RET proto-oncogene, leading to the development of tumors in the endocrine glands.

Medullary thyroid carcinoma is a hallmark feature of MEN2B, presenting in nearly all affected individuals, and it is a type of thyroid cancer arising from parafollicular C cells. The presence of marfanoid habitus, which includes features such as tall stature, long limbs, and arachnodactyly (long fingers), further distinguishes MEN2B from other conditions.

In contrast, while MEN2A also involves medullary thyroid carcinoma, it typically does not include the marfanoid habitus and presents with a different pattern of tumors, including pheochromocytomas and primary hyperparathyroidism. Conditions like adenomatous polyposis and xeroderma pigmentosum lack the specific combination of features seen in MEN2B, making this option the most accurate choice for the question provided.