An odd medical story played itself out in real time on the Internet over the past couple of weeks. An apparently bored young man decided to “take” one of his ex-girlfriend’s pregnancy tests after finding it in his medicine cabinet. The test came back positive, which the young man thought was hilarious. He told a female friend, who turned the whole thing into a rage comic, which she posted on Reddit.

Somebody called “goxilo” immediately responded to the comic with this quite somber comment: “If this is true, you should check yourself for testicular cancer. Seriously. Google it.” Soon others, including several who identified themselves as having a medical background, were also urging the young man to see a doctor.

He followed this advice — wisely, it turns out. For a doctor found that he did indeed have testicular cancer, an extremely rare type called choriocarcima. (The young man’s friend, who goes by the online name CappnPoopdeck, posted news of the diagnosis via another rage comic.)

How would a pregnancy test reveal testicular cancer? Because the test is looking for the presence in a woman’s urine of a hormone called human chorionic gonadotropin (HCG). It’s produced when an embryo has attached itself to the lining of the uterus.

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But, as Minneapolis-based freelance science writer Maggie Koerth-Baker explains in an article published online last week on Boing Boing, “HCG can show up in men, too. And when it does, bad things are happening.”

Writes Koerth-Baker:

HCG in men can be a sign of a rare (and dangerous) form of testicular cancer — choriocarcinoma. This is a cancer made up of syncytiotroblastic cells, said Katherine McGlynn, a senior investigator with the National Cancer Institutes. The tumor secretes HCG because that’s what syncytiotroblasts do. They secrete HCG. And they don’t particularly care whether they’re secreting it into a man or a woman. But how do they get into a guy, to begin with? That’s where things get really weird. The truth is that nobody is entirely certain, McGlynn told me. But there are a couple of theories. One possibility is that these syncytiotroblasts that turn cancerous were leftovers — remnants of the time when that guy was just a ball of 70-100 cells. One way or another, they persisted in his body and then started to grow out of control. The other theory: Somehow, normal cells in the man’s testes just start regressing, reverting to one of the earliest forms of cells in a human’s life cycle. Either way, one thing is certain, “It’s exactly the same cell as in the placenta,” McGlynn said. The bad news: Choriocarcinomas move really fast. They’re more common in men under 30 and the prognosis is usually bad, because most of the time nobody catches them until they’ve already spread to other parts of the body, especially the lungs.

McGlynn doesn’t believe men should run out and stock up on pregnancy tests. Choriocarcinomas are, after all, very rare. “Only about 2 men in 100,000 will get any kind of testicular cancer,” explains Koerth-Baker, “[and] pure choriocarcinomas — the dangerous kind that I’m talking about here — make up less than 1% of those diagnoses.”

But men should be aware of the signs and symptoms of testicular cancer — and seek medical care if they experience any pain, swelling or lumps in their testicles or groin area.

The young man in this story was extremely lucky. After all, what is the probability that a guy with that specific type of rare cancer would get bored one day and decide to pee on a pregnancy-test stick?

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Maybe we should ask Nate Silver.

You can read Koerth-Baker’s article on the Boing Boing website.

FYI: Koerth-Baker, who is the author of “Before the Lights Go Out: Conquering the Energy Crisis Before it Conquers Us,” will be speaking in St. Paul tomorrow night (Tuesday, Nov. 13) alongside University of Minnesota physics professor James Kakalios (“The Physics of Superheroes“) on “why conflicts of communication halt energy crisis policy from moving forward.” The event, part of the Beaker and Brush Discussions series, is co-sponsored by the Science Museum of Minnesota and the Minnesota Museum of American Art. FMI.