**Core Concept**
The ileum, the final segment of the small intestine, has distinct anatomical features compared to its proximal counterpart, the jejunum. The ileum is characterized by a unique arrangement of its blood vessels and mesenteric attachments.

**Why the Correct Answer is Right**
The ileum is distinct from the jejunum in that it has longer, more tortuous blood vessels known as vasa recta. This characteristic is due to the ileum's relatively shorter length and its need to accommodate a larger mesenteric attachment. The longer vasa recta in the ileum facilitate the increased blood supply required for the absorption of nutrients from the diet.

**Why Each Wrong Option is Incorrect**
**Option A:** The villi in the ileum are actually shorter and more club-shaped compared to the longer, slender villi found in the jejunum. This feature is not a distinguishing characteristic of the ileum.
**Option C:** While the ileum does contain a significant number of lymphoid nodules, particularly in the Peyer's patches, this is not a feature that distinguishes it from the jejunum. Both segments of the small intestine contain lymphoid tissue.
**Option D:** The mesentery of the ileum does contain a significant amount of fat, but this is not a distinguishing characteristic that sets it apart from the jejunum.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the ileum is the primary site of absorption for bile salts and vitamin B12. This is due to the presence of specific receptors and transport mechanisms in the ileal mucosa that facilitate the uptake of these nutrients.

**✓ Correct Answer: B. Long vasa recta**