**Core Concept**
The question is testing the ability to differentiate between a well-differentiated follicular carcinoma and a follicular adenoma of the thyroid gland, which is a key aspect of thyroid pathology. These two conditions are often confused due to their similar histological appearance. However, they have distinct clinical and pathological features.

**Why the Correct Answer is Right**
The key differentiating feature between a well-differentiated follicular carcinoma and a follicular adenoma is the presence of vascular invasion. Follicular carcinoma is characterized by the invasion of cancer cells into blood vessels, which allows for metastasis to distant sites. This is in contrast to follicular adenoma, which lacks vascular invasion and remains localized. The presence of capsular or vascular invasion is a hallmark of malignancy in thyroid tumors.

**Why Each Wrong Option is Incorrect**
* **Option A:** Capsular invasion alone is not sufficient to differentiate follicular carcinoma from adenoma, as some adenomas may also show capsular invasion. However, vascular invasion is a more specific indicator of malignancy.
* **Option B:** Nuclear features such as enlargement, hyperchromasia, and irregularity are not specific enough to distinguish between follicular carcinoma and adenoma.
* **Option C:** Mitotic activity and cellularity are not reliable markers for differentiating between follicular carcinoma and adenoma.

**Clinical Pearl / High-Yield Fact**
When evaluating thyroid nodules, it is essential to consider the clinical context, including the patient's age, sex, and symptoms, as well as the radiological and pathological features. The presence of vascular invasion is a critical factor in distinguishing between benign and malignant thyroid tumors.

**Correct Answer: D. Vascular invasion. Presence of vascular invasion in a well-differentiated follicular carcinoma of the thyroid gland.**