The Cottages give their findings to House, saying that abuse of amphetamines could cause neurotoxicity, which would explain Mark's symptoms. House notes that while the name on the prescription bottle "W. Brown," (WHY, Warrick?!) looks fake, the prescribing doctor is real, and just had his license revoked for writing illegal prescriptions to high-school students. How stupid do you have to be to do that, anyway? Do you not make enough as a freaking doctor to have to supplement your income from high-schoolers' petty cash? Mark is a high-school guidance counselor, and W. Brown's birthdate is consistent with one of his counselees. House thinks the pills were confiscated, not abused. Not yet, anyway, as he then pockets them. House also pooh-poohs Chase's mountain bike-to-yoga symptom-of-dementia theory, preferring to believe that it's just a symptom that he's getting old. Cameron says that the encephalitis test was negative. House says it must be Alzheimer's, then. Cameron says that the marker was negative, and she already told Stacy that Mark was free and clear. House reminds her that a missing marker doesn't mean there's no Alzheimer's (nice one, Cams!), and orders a PET scan of Mark's brain.
Foreman prepares Mark for the PET scan, which will take place in an MRI-like tube. They'll inject Mark with something that will help them map out his brain, and then Foreman will ask him memory questions so that they can see what's working up there and what isn't. Foreman walks into the booth, where House is hiding because he claims to like "pretty lights."
Mark slides into the tube, and Foreman asks questions over a speaker. They're boring, so House takes over and asks Mark about his wedding, claiming to want to find out about Mark's long-term memory and emotions. Mark's emotions are definitely working okay, as he tells House that Stacy wasn't wearing any underwear under her wedding dress, which he knows because she ripped the dress off as soon as they got in the car. For the sake of the innocent young flower girl, I hope those windows were tinted. A lot. Foreman tells House to stop it, and Mark asks House if he wants the details about their honeymoon in Paris. House says that Mark seems very defensive. Foreman says it doesn't count as paranoia when someone actually is out to get you.
House tries to get some alone time on the hospital roof, but Stacy finds him and yells at him about what he said to her husband. House says that the questions were medical in nature. Stacy says she knew House would go running up to roof like a little twelve-year-old afterward. "I haven't been up here in five years," House says, which I took to mean that he hasn't been on the roof since his leg made it difficult to take the stairs to get there. It shuts Stacy up, wonderfully, and House admits that he doesn't know what's wrong with Mark. Every test for every possible diagnosis has been negative. Stacy says she never thought House wouldn't be able to figure it out. He steps up and hugs her and she cries tearlessly. He says he hasn't given up. She pulls away and asks what they do now. House says they'll wait for something to change.