**Core Concept**
The abdominal aortic aneurysm (AAA) is a pathological dilation of the abdominal aorta, which can lead to catastrophic consequences if it ruptures. The site of rupture is crucial in determining the clinical presentation and management of the condition.

**Why the Correct Answer is Right**
The most common site of rupture of an abdominal aortic aneurysm is laterally into the left retroperitoneum. This is due to the anatomical characteristics of the aorta and its surrounding structures. The aorta is a retroperitoneal structure that is situated behind the peritoneum, and it is most prone to rupture at the points where it is weakest, which is usually at the level of the left renal artery. The lateral location of the rupture is due to the fact that the aorta is subjected to the highest stress in this area, which is exacerbated by the presence of the left renal artery and the surrounding retroperitoneal fat.

**Why Each Wrong Option is Incorrect**
**Option B:** Rupture into the right retroperitoneum is less common than into the left retroperitoneum, as the right renal artery arises from the aorta at a more acute angle, which reduces the stress on the aorta in this region.

**Option C:** Posterior rupture into the posterior retroperitoneum is rare, as the aorta is relatively protected by the surrounding vertebral bodies and the psoas major muscle.

**Option D:** Anterior rupture into the peritoneum is extremely rare, as the aorta is surrounded by a thick layer of retroperitoneal fat and the peritoneum, which provides a degree of protection against rupture.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the majority of abdominal aortic aneurysms are asymptomatic until rupture, and therefore, regular screening and surveillance are crucial in detecting and managing these lesions before they become catastrophic.

✓ Correct Answer: A. Laterally into the Left Retroperitoneum