**Core Concept**
Mesenteric tumors are a rare group of neoplasms that arise from the mesentery, a fold of tissue that attaches the intestine to the back of the abdominal wall. These tumors can be benign or malignant and are often characterized by their unique location and potential to cause abdominal symptoms.

**Why the Correct Answer is Right**
The correct answer is B. Usually cystic, because mesenteric tumors are often cystic in nature, particularly the benign ones. These cystic tumors are typically of mesothelial origin and are also known as mesenteric cysts. They can grow to varying sizes and may cause abdominal discomfort, pain, or other symptoms due to their location and size. The cystic nature of these tumors is a result of the mesothelial cells lining the cyst wall and producing a fluid-filled cavity.

**Why Each Wrong Option is Incorrect**
**Option A:** Usually solid mesenteric tumors are less common, and many of them are benign. However, when they are malignant, they are often more aggressive and have a worse prognosis.

**Option C:** While some mesenteric tumors can be malignant, they are not typically highly malignant. The malignancy potential varies depending on the type of tumor, but many mesenteric tumors are benign.

**Option D:** Highly vascular mesenteric tumors are not a characteristic of these neoplasms. While some tumors may have a rich blood supply, this is not a hallmark feature of mesenteric tumors.

**Clinical Pearl / High-Yield Fact**
A key feature of mesenteric cysts is their tendency to cause abdominal symptoms due to their location and size. These symptoms can include abdominal pain, discomfort, or even obstruction of the intestine. A high index of suspicion and imaging studies are often necessary to diagnose these rare tumors.

**✓ Correct Answer: B. Usually cystic**