Which of the following is a marker for medullary thyroid cancer?

Calcitonin is the correct marker for medullary thyroid cancer because it is a hormone produced by the parafollicular cells (also known as C cells) of the thyroid gland. Medullary thyroid cancer arises from these C cells, and elevated levels of calcitonin in the blood are commonly associated with the presence of this type of cancer. Measuring calcitonin can be a useful diagnostic and monitoring tool for patients with or suspected of having medullary thyroid carcinoma.

In contrast, insulin is primarily associated with the pancreas and is not relevant to thyroid cancer diagnosis. Prolactin is a hormone produced by the pituitary gland and is unrelated to thyroid cancer. CEA, or carcinoembryonic antigen, is a marker more commonly associated with certain types of gastrointestinal cancers and some other malignancies, but it is not specific to medullary thyroid cancer. Therefore, calcitonin serves as a specific and relevant biomarker for medullary thyroid cancer, making it the correct choice.