Episode 45 of "House"
s02e23 Who's Your Daddy?

A 16-year-old female Hurricane Katrina victim suffering from hallucinations about the tragedy is brought to House by his ex-bandmate Crandall (guest star D.B. Sweeney, who tells House he recently found out she's his daughter. House takes the case despite feeling more pain than usual in his leg. He wants to run a paternity test to make sure his friend is not being scammed, but Crandall refuses. As the team treats the patient, House still has his suspicions, and when her treatment requires a bone marrow transplant, Crandall offers himself up, but must be tested to see if he's a match (while still refusing the paternity test). As House works his way through the girl's lies to find clues to diagnose and treat her, he is forced to tell some lies of his own. Meanwhile, Cuddy asks House to review two medical files, and it doesn't take long for him to figure out that she's looking for a sperm donor. But House takes the search a step further and sets Cuddy up for a big surprise.