Final answer:
Assessment of bruit on the left forearm is the observation that indicates the arteriovenous (AV) fistula is patent.
Step-by-step explanation:
The observation by the nurse that indicates the arteriovenous (AV) fistula is patent is assessment of bruit on the left forearm. A bruit is a swishing sound that can be heard with a stethoscope over an AV fistula or graft. It indicates that blood flow is present and moving through the fistula.