Which of the following is a severe adverse reaction associated with aminoglycoside therapy?

A. Rash is generally a mild adverse effect and does not indicate a severe reaction. B. Nausea and vomiting are common side effects but not classified as severe reactions. C. Ototoxicity is a severe adverse reaction associated with aminoglycoside therapy, potentially leading to hearing loss and balance issues. Monitoring for this side effect is crucial during treatment. D. Headache can occur but is not considered a severe adverse reaction compared to ototoxicity.