Like last week’s episode, this week’s House M.D. stuck to the formula that has worked so well for the series these past 6.5 seasons: medical mystery, a flurry of possible diagnoses — more than any episode this season, I think — and then a twist at the end. like Law & Order, one of the longest-running TV franchises, House M.D. never strays too far from its procedural roots.

But before we get to the medicine (with all the possible diagnoses in bold below, as usual), a spoiler alert: if you didn’t watch the episode, “Carrot or Stick,” put on a hospital gown and fire up the DVR before reading on.

The 10th installment of Season 7 begins at a boot camp for wayward boys. a drill instructor, Driscoll, is screaming at one of the boys, whose name is Landon, to finish an obstacle course even though Landon is exhausted and has cut his forehead. You think the kid might collapse, but true to House M.D. form, it is the drill instructor who falls in the mud. He says he has excruciating back pain.

House brings the case to the team and cites another symptom: “38-year-old former Marine. Came into the ER with back pain. Now his bladder aches as much as his back because he can’t empty it.”

Chase wonders if Driscoll has taken anticholinergics, which reduce muscle spasm but would also make it harder to push urine from his bladder, or antihistamines, which in large doses could cause both symptoms — setting up two possible poisoning diagnoses.

No, says House, and the ER ruled out enlarged prostate and spinal injury. Masters suggests steroid use since Driscoll is in suspiciously good shape for his age and is also aggressive with the kids. But House calls her diagnosis “moronic” since there’s no sign of hormonal imbalance.

Driscoll’s urine is clean for bacteria, so it’s not a bacterial infection, but Foreman says he might have neurogenic bladder, a condition in which the bladder doesn’t function properly because of a neurological problem. your bladder is a complex device that requires muscles and nerves to work in concert so that you can hold urine until you reach a toilet. a neurological condition could disrupt muscle control and nerve impulses.

But something would have to set off neurogenic bladder, which is a symptom, not a diagnosis. Chase suggests a spinal cord tumor or cerebral palsy, but Foreman says House will want to check for syphilis first.

When Masters asks why, Taub chimes in, “Rare complication of an embarrassing illness — practically a House specialty.” Masters draws blood to test for syphilis, but when she does, Driscoll attacks her and says she’s trying to kill him. that offers another symptom: psychosis. At the differential-diagnosis meeting, Foreman goes back to poisoning; this time he suggests the culprits may be tricyclic antidepressants or Ritalin. Masters says an overdose of wormwood (the psychoactive substance in absinthe) or jimson weed (which grows wild outdoors and could be on the obstacle course) could also cause the symptoms.

But Chase notes that Driscoll probably would have mentioned using wormwood or jimson weed since it would take enormous doses to make him so sick.

Masters then offers a twist on the poisoning diagnosis: “Maybe one of those kids at the camp got tired of Driscoll’s lessons and tried to poison him.”

Masters and Foreman go the camp and find a boy who hasn’t been taking his antihistamines. Landon admits using the other boy’s antihistamines to poison Driscoll. Back at the hospital, they give Driscoll IV fluids to flush the drugs from his system. But as they do, he complains of a new symptom: his heart is racing.

A second differential meeting brings a flurry of possible diagnoses: Taub suggests mastocytosis, a skin disease that can lead to tachycardia (rapid heartbeat) but wouldn’t explain the other symptoms. Masters offers insulinoma, a tumor of the pancreas that causes low blood sugar, which in turn could cause psychiatric symptoms and, possibly, Driscoll’s shaky heart. But what about the back pain?

Chase throws in pheochromocytoma, a rare adrenal-gland tumor that causes the gland to release too much epinephrine and norepinephrine, hormones that regulate heart rate. House, who likes rare diseases better than well-known ones, opts for Chase’s idea, but it turns out Driscoll’s adrenal gland is fine.

The team seems at a loss when the boy that Driscoll was bullying at the top of the show, Landon, comes in with the same symptoms. At yet another differential meeting (this episode was clearly for the show’s physician fans), Masters goes back to infection: “The camp is in the woods,” she points out. “There are tons of opportunities for infection, and not every person will pick up. Legionellosis [legionnaires' disease] from water. Toxoplasmosis from dirt. Brucellosis from animals.”

But legionnaires’ usually presents with aches and fever, not back pain, psychosis, and tachycardia. Toxoplasmosis, which is spread by the parasite toxoplasma, is usually a food-borne illness that would cause aches and tender lymph nodes. Usually spread by animals, brucellosis would cause flu-like symptoms.

Foreman proposes Lyme disease, which causes widely varying symptoms depending on the individual bitten by an infected tick. Masters points out that neither patient has a rash or a tick bite, but Foreman answers, correctly, that at least 40% of Lyme patients don’t develop a rash. “And a bite is easy to miss,” he says.

They start Driscoll on an antibiotic (doxycycline), but soon he has another symptom: his leg muscles feel like they are in a vise. This leads to a fourth differential meeting. “The leg pain was caused by a muscle cramp from low sodium, which was caused by kidney dysfunction,” Foreman posits.

House wonders what could cause kidney dysfunction on top of everything else.

“Arsenic poisoning,” says Masters.

“If it was arsenic, they’d be puking and pooping,” replies House.

Sticking with the poisoning theory, Taub notes that compromised kidney function that is secondary to bladder issues would point to botulism toxin, which lives in soil. “Got into the kid’s bloodstream through the cut on his forehead; got into the drill instructors’ through some abrasions on his feet,” Taub concludes.

Everyone is reaching now. House suggests Wegener’s granulomatosis, a rare disorder that restricts blood flow to various organs, especially the kidneys, and causes granuloma, inflammation so severe that destroys normal tissue (explaining the pain). Masters mentions that she and Foreman saw some old generator batteries at the camp. It’s a weak theory — it doesn’t explain the psychosis — but it’s all they have.

Finally, when House is in the infirmary, he has his aha moment. He’s about to numb a patient’s badly scraped leg with lidocaine when he realizes Landon, the boy from the camp, probably got lidocaine for the cut on his head. He goes to Driscoll and Landon and explains that the lidocaine Landon got and the antihistamines with which he dosed Driscoll set off a genetic illness called variegate porphyria (final diagnosis — thank god!). Variegate porphyria (see more info here) causes the body to accumulate porphyrins, which are found all over the body but can be harmful when they accumulate. Porphyria can effect the nervous system (explaining the urine retention) and can cause a wide array of other symptoms depending on the individual.

Because Landon turns out to be Driscoll’s son, they had identical symptoms. Masters says the drug Hematin will control their condition, although they will have to get liver transplants to be cured.

Overall, “Carrot or Stick” was my least favorite episode of the season. the father-son revelation simply didn’t rescue this episode from its overreliance on differential meetings, and the way House arrived at the final diagnosis was unconvincing. Still, House M.D. is back in two weeks with another Candice Bergen episode that is being billed as a mold-breaker. see you then!