**Core Concept**
An abdominal aortic aneurysm (AAA) is a localized dilation of the abdominal aorta, typically greater than 3 cm in diameter. In this scenario, the patient has a large AAA of 40 mm, which poses a significant risk of rupture.

**Why the Correct Answer is Right**
The next immediate step in managing a large abdominal aortic aneurysm is to perform a computed tomography (CT) angiogram to assess the aneurysm's size, shape, and location. This imaging will help determine the presence of any complications such as mural thrombus, intramural hemorrhage, or aortic dissection. Additionally, it will provide valuable information to plan for potential surgical repair or endovascular aneurysm repair (EVAR) if necessary.

**Why Each Wrong Option is Incorrect**
**Option A:** Immediate surgery is not always necessary for a large AAA, as the decision to operate depends on the patient's risk factors, aneurysm size, and growth rate. A CT angiogram is required to assess the aneurysm before planning any intervention.
**Option B:** Serial ultrasound monitoring may be used for smaller aneurysms, but a large AAA of 40 mm requires more urgent evaluation and planning for potential repair.
**Option C:** Aortic aneurysm repair is a significant surgical procedure, and it is not a suitable option without proper imaging and preoperative planning.

**Clinical Pearl / High-Yield Fact**
The size and growth rate of an abdominal aortic aneurysm are crucial factors in determining the need for surgical repair. Aneurysms with a diameter of 5.5 cm or greater are generally recommended for surgical intervention.

**Correct Answer: C. Aortic aneurysm repair is a significant surgical procedure, and it is not a suitable option without proper imaging and preoperative planning.