**Question:** Aeriovenous fistula can safely be ligated if the following is positive:

A. Presence of collateral circulation
B. Presence of shunt
C. Presence of aneurysm
D. Absence of renal failure

**Core Concept:**
Aerovenous fistula is a rare condition where an artery and vein are fused together, typically due to a congenital defect or trauma. When considering ligation of an aerovenous fistula, it is essential to assess the potential complications and ensure that the procedure can be done safely. The ligation of an aerovenous fistula should be done only when the associated complications are managed effectively.

**Why the Correct Answer is Right:**
The correct answer is "D. Absence of renal failure." Ligation of an aerovenous fistula can be considered safe in patients without renal failure because renal perfusion depends on the systemic blood pressure, which is maintained when the fistula is ligated. In contrast, renal failure patients may experience acute renal failure due to decreased renal perfusion following ligation.

**Why Each Wrong Option is Incorrect:**
A. Presence of collateral circulation (Option A): Collateral circulation is not a contraindication for ligation, but rather, it is a compensatory mechanism that helps maintain renal perfusion. If collateral circulation is present, it demonstrates the kidneys' ability to adapt to reduced blood flow, making ligation safer.

B. Presence of shunt (Option B): A shunt is another compensatory mechanism that allows blood to bypass the affected kidney. Shunt presence does not directly contraindicate ligation but may indicate more significant renal parenchymal damage, making the procedure more challenging and potentially increasing the risk of complications.

C. Presence of aneurysm (Option C): An aneurysm indicates significant arterial wall dilation, which can lead to increased risk of complications during ligation. Moreover, aneurysm rupture is a known complication after renal artery ligation. Hence, the presence of aneurysm is a contraindication for ligation.

**Clinical Pearl:**
In summary, ligation of an aerovenous fistula can be considered safe in patients without renal failure, collateral circulation, shunt, or aneurysm, as these factors indicate compensatory mechanisms or increased risk of complications.

**Why Each Wrong Option is Incorrect:**
A. Presence of collateral circulation: Collateral circulation is not a direct contraindication but shows renal adaptation to reduced blood flow, which may complicate ligation and increase the risk of complications.
B. Presence of shunt: A shunt indicates significant renal parenchymal damage, increasing the risk of complications during ligation and potentially causing aneurysm formation or rupture.
C. Presence of aneurysm: An aneurysm indicates arterial wall dilation, which increases the risk of complications during ligation and poses a potential rupture risk.