**Core Concept**
The primary distinction between follicular carcinoma and follicular adenoma of the thyroid gland lies in their potential for metastasis and invasion into surrounding tissues. Follicular carcinomas exhibit a higher propensity for capsular and vascular invasion, whereas follicular adenomas are generally benign and well-circumscribed.

**Why the Correct Answer is Right**
The key differentiating feature between follicular carcinoma and follicular adenoma is the presence of capsular and vascular invasion. Capsular invasion refers to the tumor's ability to infiltrate through the capsule that surrounds the thyroid gland, while vascular invasion involves the penetration of tumor cells into blood vessels. This invasion allows follicular carcinomas to metastasize to distant sites, such as the lungs and bones, whereas follicular adenomas do not exhibit this behavior. The ability to identify vascular and capsular invasion under microscopic examination is crucial for diagnosing follicular carcinoma.

**Why Each Wrong Option is Incorrect**
**Option A:** Histological patterns of tumor growth, such as papillary or follicular patterns, are not sufficient to distinguish between follicular carcinoma and follicular adenoma. Both types of tumors can exhibit similar histological features.

**Option B:** The presence of lymphocytic infiltration or hemorrhage within the tumor is not a reliable indicator of malignancy. These features can be seen in both benign and malignant thyroid lesions.

**Option C:** The size of the tumor is not a reliable criterion for distinguishing between follicular carcinoma and follicular adenoma. Large tumors can be benign, while small tumors can be malignant.

**Clinical Pearl / High-Yield Fact**
When evaluating thyroid lesions, it is essential to examine the tumor's margins and the presence of capsular and vascular invasion, as these features are critical for distinguishing between follicular carcinoma and follicular adenoma.

**Correct Answer: C. Vascular and capsular invasion.**