Bernie Sanders is arguing that medical bills cause 500,000 bankruptcies a year, and thus we need his national health insurance plan. Megan McArdle, who has been covering this for years, continues to do the thankless work of correcting the misunderstandings behind this argument. The key point: If you use a definition of “medical bankruptcy” expansive enough to yield the huge numbers that single-payer advocates use, then you have to count a lot of bankruptcies that a government-run health-care system won’t do much if anything to prevent. As she notes, “if you get sick and can’t work, and you have a lot of debt, things go south pretty quick.”