**Core Concept**
In pediatric surgery, differential diagnoses for appendicitis in children are crucial for accurate diagnosis and management. Mesenteric lymphadenitis is a condition characterized by inflammation of the lymph nodes in the mesentery, a fold of tissue that attaches the intestine to the back of the abdominal wall.

**Why the Correct Answer is Right**
Mesenteric lymphadenitis is a common differential diagnosis for appendicitis in children due to its similar clinical presentation, which includes abdominal pain, fever, and leukocytosis. The inflammation of the lymph nodes in the mesentery can mimic the symptoms of appendicitis, making it a challenging diagnosis to distinguish. The condition is often caused by viral or bacterial infections, which can lead to the activation of lymphoid tissue and subsequent inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastroenteritis is a condition that affects the stomach and intestines, causing symptoms such as diarrhea, vomiting, and abdominal cramps. While it can present with abdominal pain, it is not a common differential diagnosis for appendicitis in children.
**Option C:** Intussusception is a condition where a part of the intestine slides into another part, causing bowel obstruction and abdominal pain. It is a serious condition that requires prompt medical attention, but it is not typically a differential diagnosis for appendicitis.
**Option D:** Meckel's diverticulitis is a condition that affects the Meckel's diverticulum, a small pouch in the small intestine. It can cause abdominal pain, but it is not a common differential diagnosis for appendicitis in children.

**Clinical Pearl / High-Yield Fact**
In children, the clinical presentation of appendicitis can be atypical, making differential diagnoses such as mesenteric lymphadenitis more likely. A high index of suspicion and a thorough physical examination are essential in distinguishing between these conditions.

**Correct Answer:**
✓ Correct Answer: B. Mesenteric lymphadenopathy