Derek later recruits Lexie for his unauthorized "rogue surgery" on hospital technician Isaac's (Faran Tahir) "inoperable" tumor, with Lexie acting as Derek's own caregiver during the long and exhausting procedure. Lexie is supported by Mark and, to avoid having to leave the operating room, resorts to using a diaper so that she can sufficiently hydrate herself before and during the surgery. Near the year's end, Mark and Lexie receive a shock when a pregnant teenage girl named Sloan Riley (Leven Rambin) shows up claiming to be Mark's daughter, and her parentage is soon confirmed. Mark quickly agrees to let Sloan and the baby move in permanently without consulting Lexie, resulting in her ending their relationship as she feels she is still too young to be a grandmother. An upset and drunk Lexie then engages in a one-night stand with Alex but feels guilty and confesses to Mark, who becomes furious with her despite also admitting that he had a one-night stand with Addison Montgomery (Kate Walsh). Although she secretly suppresses her feelings for Mark, Lexie and Alex assume a casual relationship that Lexie flaunts to make Mark jealous. Lexie later breaks down in front of Meredith when Alex tells her that Mark has started dating Teddy Altman (Kim Raver). After Sloan gives her baby up for adoption and leaves Seattle, however, Mark approaches Lexie and confesses his love to her. A stunned Lexie hesitantly states that Alex is now her boyfriend, to which Mark replies, "I know. I'm just saying you could have a husband."