How would you fix this sentence? He passed about a cup of bright red blood per rectum, which brought him to the emergency room.

It looks o.k. to me.

Well, passing blood did not suddenly transport him to the emergency room as in, "Beam me up, Scotty."

It's implied that he went to the emergency room, so the sentence is okay as stated. But one might write: "He passed about a cup of bright red blood from his rectum, which prompted him to go to the emergency room."

To fix this sentence, you can make a few adjustments:

"He passed approximately a cup of bright red blood per rectum, which led him to the emergency room."

Explanation:
1. Change "about" to "approximately" for more precision.
2. Use "led" instead of "brought" to add clarity to the sentence structure.