A 35-year-old woman presents with a tender neck and an enlarged thyroid. An inhomogeneous thyroid with irregular borders is seen on sonogram. This is most characteristic of which condition?

The presence of a tender neck and an enlarged thyroid, along with the ultrasound findings of an inhomogeneous thyroid with irregular borders, strongly indicates Hashimoto's Thyroiditis. This autoimmune condition causes chronic inflammation of the thyroid gland, leading to its enlargement and the typical ultrasound appearance characterized by heterogeneous echogenicity and irregular contours.

In Hashimoto's Thyroiditis, the irregular borders and inhomogeneity reflect the infiltrative nature of the autoimmune process, which results in both lymphocytic infiltration and variable fibrosis throughout the gland. Over time, this results in altered architectural integrity, contributing to the irregular appearance on imaging. The tenderness may be a result of acute exacerbations or a recent increase in inflammatory activity.

In contrast, while malignancies can also present as an enlarged thyroid with irregular borders, they often have other specific sonographic features such as microcalcifications or significant vascularity that help differentiate them from benign conditions. Graves' Disease typically presents with a diffusely enlarged, homogeneous goiter and is less likely to be tender. Adenomatous Hyperplasia may lead to an enlarged gland but usually demonstrates a more homogeneous appearance on ultrasound. Thus, the overall clinical picture and imaging findings align most closely with Hashimoto's Thyroiditis.