USMLE Step 1 Practice Exam 2025 – 400 Free Practice Questions to Pass the Exam

Follicular adenoma is characterized by being surrounded by a complete capsule, distinguishing it as a benign tumor of the thyroid. This encapsulation is significant because it indicates that the tumor is confined and does not invade surrounding tissues, which is a hallmark of benign neoplasms. The presence of a complete capsule helps in the pathologic differentiation between follicular adenoma and follicular carcinoma, the latter of which typically exhibits vascular or capsular invasion.

The presence of psammoma bodies, Orphan Annie nuclei, and rapid hematogenous spread are features more commonly associated with different conditions. Psammoma bodies are calcified structures seen in certain papillary carcinomas and other neoplasms but are not a feature of follicular adenoma. Orphan Annie nuclei are characteristic of papillary thyroid carcinoma, not follicular adenoma, as they represent thyroid cells that have undergone atypical changes. Rapid hematogenous spread is more typical of malignant tumors rather than benign ones like follicular adenoma, which demonstrates a more localized growth pattern. Thus, the defining feature of follicular adenoma is indeed the complete capsule that encases the tumor.