**Core Concept**
The rotavirus vaccine is a live, attenuated vaccine designed to protect against severe gastroenteritis caused by rotavirus. The vaccine's adverse effects are typically mild and self-limiting, but in rare cases, they can lead to more severe complications.

**Why the Correct Answer is Right**
The rotavirus vaccine has been associated with an increased risk of intussusception, a serious gastrointestinal condition where a part of the intestine telescopes into another part. This is thought to occur due to the presence of the live, attenuated virus in the vaccine, which can cause the intestines to become inflamed and lead to intussusception. The exact mechanism is not fully understood, but it is believed to involve an abnormal immune response to the vaccine virus.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastroenteritis - This is the condition that the rotavirus vaccine is designed to prevent, not a complication of the vaccine itself.
**Option B:** Guillain-Barré Syndrome - This is a rare autoimmune disorder that has been linked to certain vaccines, but not the rotavirus vaccine.
**Option C:** Anaphylaxis - This is a severe, life-threatening allergic reaction that can occur with any vaccine, but it is not a specific complication associated with the rotavirus vaccine.

**Clinical Pearl / High-Yield Fact**
The risk of intussusception associated with the rotavirus vaccine is highest in the first week after vaccination, and it is more common in infants who receive the vaccine at an older age.

**Correct Answer:** C. Intussusception.