Although initially happy in her relationship with Jackson, Lexie grows increasingly distraught and frustrated when she discovers that Mark has started dating an ophthalmologist named Julia. When she sees Mark and Julia flirting during a charity softball match, Lexie's jealousy gets the better of her and she throws a ball at Julia, injuring the latter's chest. Jackson senses that Lexie is still in love with Mark and ends their relationship. Lexie begins working under Derek's service and becomes increasingly proficient in neurosurgery, helping Derek with a set of "hopeless cases" – high risk surgeries for patients who had otherwise run out of options. During a surgery, Derek is called away on an emergency, leaving Lexie and Meredith to carry out the procedure on their own. Though Derek had instructed them to merely reduce the patient's brain tumor, Meredith allows Lexie to remove it completely, despite not being authorized by either the patient or Derek to do so. The sisters celebrate the successful surgery but Lexie is devastated when she discovers that the patient suffered severe brain damage, thus losing the ability to speak.