Along with removing drugs from the body, the kidney also

Kidneys are an extremely important part in the full functionality of the body they not only eliminate toxins but they regulate the levels of salt, potassium, and acid. They also produce hormones that helo other organs to function.

The kidneys are involved in several processes like removing waste products from the body, removing drugs from the body, balancing the body's fluids, releasing  hormones that regulate blood pressure , producing an active form of vitamin D that promotes strong, healthy bones and controlling the production of red blood cells

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the answer to "what does the narrator realize by the end of the story" is b.