What is the definition of an arteriovenous fistula?

• A. Abnormal small direct connections between an artery and vein so that it bypasses a capillary bed
• B. A malformation of veins with no arterial input
• C. A localized dilation of an artery
• D. Abnormal direct connections between an artery and vein so that it bypasses a capillary bed

Answer: (D)

An arteriovenous fistula is an abnormal direct connection between an artery and a vein that bypasses the capillary bed, creating a high-flow arteriovenous shunt. This means arterial blood dumps straight into the venous system without passing through the normal microcirculation, which can lead to enlarged veins and increased venous return. The essential idea is the direct artery-to-vein link without an intervening capillary network. The other descriptions don’t capture this direct shunt: a venous malformation has no arterial input, and an arterial dilation is not a shunt. The word “small” isn’t a defining part of the concept, so it’s not needed.