**Core Concept**
Elective repair of an abdominal aortic aneurysm (AAA) is considered when the aneurysm is asymptomatic and at a size that poses a significant risk of rupture. The optimal diameter threshold for elective repair is a critical decision point, balancing the risk of rupture against the risks of surgery.

**Why the Correct Answer is Right**
The recommended diameter threshold for elective repair of an AAA is generally accepted to be 55 mm. This is based on the risk of rupture, which increases exponentially with aneurysm size. Aneurysms smaller than 55 mm have a low risk of rupture, whereas those larger than this threshold have a significantly higher risk. The decision to repair an aneurysm is also influenced by factors such as growth rate, patient comorbidities, and life expectancy.

**Why Each Wrong Option is Incorrect**
* **Option A:** 40 mm - Aneurysms smaller than 55 mm have a low risk of rupture, and elective repair is not typically recommended at this size.
* **Option B:** 60 mm - While aneurysms larger than 55 mm have a higher risk of rupture, elective repair is not universally recommended at 60 mm without considering other factors such as growth rate and patient comorbidities.
* **Option C:** 70 mm - An aneurysm size of 70 mm is significantly larger than the recommended threshold for elective repair, and immediate surgery is often considered at this size.

**Clinical Pearl / High-Yield Fact**
The growth rate of an AAA is an important factor in determining the need for elective repair. Aneurysms that grow rapidly (e.g., > 0.5 cm/year) may require earlier intervention, regardless of the absolute diameter.

**Correct Answer: D. 55 mm**