USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

Follicular adenoma is the correct answer because it commonly presents as a solitary "cold" nodule and is surrounded by a complete capsule. This differentiation is important as follicular carcinoma, while it may also present as a solitary nodule, does not have a complete capsule surrounding it, making option B incorrect. Papillary adenocarcinoma typically presents with a mixture of solid and cystic components, not as a solitary nodule with a complete capsule. Riedel thyroiditis is characterized by extensive fibrosis involving the thyroid gland, not by a solitary nodule with a complete capsule.