What radiopharmaceutical does a parathyroid adenoma concentrate?

The correct choice is Tc99-Sestamibi, which is a radiopharmaceutical commonly used in the evaluation of parathyroid adenomas. This compound has a high affinity for the mitochondria within hyperfunctioning parathyroid tissue, allowing it to accumulate in these areas during imaging. This attribute is particularly useful for localizing parathyroid adenomas as it helps in differentiating these tissues from the surrounding normal glandular tissue and helps assess the presence of hyperparathyroidism.

In contrast, the other radiopharmaceuticals listed do not have the same specificity for parathyroid tissue. 99mTc-exametazime is primarily used for brain perfusion studies and has little relevance in the context of parathyroid imaging. 201Tl-thallous chloride is also primarily used in cardiac imaging and has different kinetic properties that do not favor the concentration in parathyroid tissues. 99mTc-medronate is utilized mainly for bone imaging and is not suitable for visualizing parathyroid adenomas. Therefore, among the offered choices, Tc99-Sestamibi is the only radiopharmaceutical that effectively concentrates in parathyroid adenomas, making it the best answer for this question.