**Core Concept**
The patient's symptoms, including acute abdominal pain, pyrexia, and localized peritonism in the right iliac fossa (RIF), are suggestive of acute appendicitis. This condition is characterized by inflammation of the appendix, a small, tube-like structure attached to the large intestine. The causative cell involved in the pathogenesis of acute appendicitis is a key aspect of understanding the underlying mechanism.

**Why the Correct Answer is Right**
The causative cell involved in acute appendicitis is the **neutrophil**, a type of white blood cell that plays a crucial role in the inflammatory response. Neutrophils are attracted to the site of inflammation, where they attempt to eliminate the causative agent, in this case, an obstruction of the appendiceal lumen. The neutrophilic response leads to the release of various enzymes and cytokines, resulting in the characteristic symptoms of acute appendicitis, including pain, fever, and localized peritonism. The neutrophils also contribute to the formation of a fibrinopurulent exudate, which can lead to the rupture of the appendix and subsequent peritonitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the causative cell involved in acute appendicitis is not the lymphocyte, which plays a key role in adaptive immunity but is not directly involved in the acute inflammatory response.

**Option B:** This option is incorrect as the eosinophil is a type of white blood cell involved in the response to parasites and allergic reactions, but it is not the primary cell involved in the pathogenesis of acute appendicitis.

**Option C:** This option is incorrect as the monocyte is a type of white blood cell that matures into macrophages, which play a role in the resolution of inflammation and the clearance of pathogens, but it is not the primary cell involved in the acute inflammatory response of acute appendicitis.

**Option D:** This option is incorrect as the platelet is a type of blood cell involved in blood clotting, but it is not directly involved in the pathogenesis of acute appendicitis.

**Clinical Pearl / High-Yield Fact**
Neutrophils are the primary white blood cells involved in the acute inflammatory response, and their activation and recruitment to the site of inflammation are key events in the pathogenesis of acute appendicitis. A high index of suspicion for acute appendicitis is essential in patients presenting with right lower quadrant pain and fever, as prompt diagnosis and treatment can prevent complications such as peritonitis and appendiceal rupture.

**Correct Answer:** C. Neutrophil.