yī / shēng / gěi / tā / kāi / le / yào / yǐ / fáng / zhǐ / xuè / guǎn / lǐ / xíng / chéng / níng / xuè

医生给他开了药，以防止血管里形成凝血。

The doctor prescribed medicine to prevent blood clots from forming in his blood vessels.