**Core Concept**
The formation of an intraperitoneal abscess involves the accumulation of pus within the peritoneal cavity, typically resulting from a breach in the integrity of the gastrointestinal tract or the spread of infection from adjacent organs. The peritoneal cavity is a potential space that can accommodate fluid and gas, making it a common site for abscess formation.

**Why the Correct Answer is Right**
The commonest site of an intraperitoneal abscess is the pelvic region, particularly in the area of the pouch of Douglas. This is due to the natural gravitational flow of infected fluid and the relative sequestration of the pelvic region, which can impede the spread of infection. The pouch of Douglas is a peritoneal recess located between the rectum and the uterus in females, and between the rectum and the bladder in males. The presence of a pelvic abscess can lead to symptoms such as pelvic pain, fever, and changes in bowel habits.

**Why Each Wrong Option is Incorrect**
* **Option A:** While the right lower quadrant of the abdomen can be a site for abscess formation, particularly in the case of appendicitis, it is not the commonest site for an intraperitoneal abscess.
* **Option B:** The left upper quadrant of the abdomen is not typically associated with the formation of intraperitoneal abscesses.
* **Option D:** The retroperitoneal space is located behind the peritoneum and is not typically associated with the formation of intraperitoneal abscesses.

**Clinical Pearl / High-Yield Fact**
The location of an intraperitoneal abscess can often be predicted based on the underlying cause of the infection, with abscesses often forming in the area of the gastrointestinal tract that is most affected.

**Correct Answer:** C.