This paper introduces LEA-Dialog, a multi-turn diagnostic dialogue dataset for lower-extremity arteriovenous diseases, together with a carefully developed diagnostic handbook and a process-aligned agentic framework for structured outpatient diagnosis. The dataset provides stage annotations for each turn and guideline-grounded probability trends, enabling evaluation beyond final diagnostic accuracy. Experiments show that the framework improves reasoning stability and reduces drift across both online and offline LLMs, with particularly large gains for smaller offline models.