Final answer:
St. Benedict of Nursia is credited with starting the first medieval monastery and composing the monastic Rule that became a foundational guide for monastic life.
Step-by-step explanation:
The person credited with starting the first medieval monastery is St. Benedict of Nursia. Around 529, this influential monastic leader composed the Rule of St. Benedict, a guidebook for monastic life that laid out the daily lives of monks including prayer, study, and useful work. St. Benedict's rules emphasized moderation, prayer, manual labor, and self-sufficiency, making monasteries not only spiritual centers but also economic entities within the medieval world.