**Core Concept**
Mesenteric cysts are rare, benign or malignant, fluid-filled lesions that occur within the mesentery, a fold of tissue that attaches the intestines to the back of the abdominal wall. These cysts can cause intestinal obstruction, hemorrhage, or other complications if they become large enough.

**Why the Correct Answer is Right**
The correct answer will be the statement that does not apply to mesenteric cysts. Mesenteric cysts can be classified into different types, including enteric, lymphatic, and enterogenous cysts. They can also be benign or malignant. The majority of mesenteric cysts are benign, but they can cause significant symptoms and complications if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** Mesenteric cysts are rare, but they can occur in any age group. This statement is true, so it is not the correct answer.
**Option B:** Mesenteric cysts can cause intestinal obstruction, hemorrhage, or other complications if they become large enough. This statement is true, so it is not the correct answer.
**Option C:** Mesenteric cysts are always benign. This statement is false, as some mesenteric cysts can be malignant. Therefore, this is the correct answer.

**Clinical Pearl / High-Yield Fact**
It is essential for clinicians to consider mesenteric cysts in the differential diagnosis of abdominal masses or masses in the mesentery, especially in patients with non-specific symptoms such as abdominal pain or weight loss.

**Correct Answer: C. Mesenteric cysts are always benign.