**Core Concept**
The formation of an intraperitoneal abscess following peritonitis involves the accumulation of pus in the peritoneal cavity due to a localized collection of bacteria and inflammatory cells. This process is a result of the body's attempt to contain and isolate the infectious process.

**Why the Correct Answer is Right**
The most common location for an intraperitoneal abscess is the pelvis, where the peritoneal cavity is in close proximity to the pelvic organs. The accumulation of pus in this region can be attributed to the gravitational pooling of infected fluid, as well as the fact that the pelvic organs, such as the appendix, are more prone to perforation and subsequent infection. The pelvic location of the abscess also makes it more difficult to diagnose and treat, as it can be obscured by overlying bowel loops and pelvic structures.

**Why Each Wrong Option is Incorrect**
**Option A:** The right subphrenic space is another possible location for an intraperitoneal abscess, but it is less common than the pelvic location.

**Option B:** The right upper quadrant of the abdomen is not a typical location for an intraperitoneal abscess following peritonitis.

**Option C:** The left upper quadrant of the abdomen can be a location for an intraperitoneal abscess, but it is less common than the pelvic location.

**Clinical Pearl / High-Yield Fact**
When suspecting an intraperitoneal abscess, it is essential to remember that the most common location is the pelvis, and that imaging studies such as CT scans or ultrasound should be used to confirm the diagnosis.

**Correct Answer:** C.