NEW YORK (CNNMoney) -- Pop quiz: What's the fastest-growing portion of the federal budget this year?
There are a number of good candidates. Maybe it's Social Security. We know the baby boomers are retiring. Is it defense? The Pentagon is fighting wars in Afghanistan and Iraq, after all.
Maybe it's the ongoing cost of those bailouts that Congress passed more than two years ago.
OK, suspense over. It's the interest on the national debt, according to the Congressional Budget Office.
Every month, CBO prepares a two-page balance sheet that summarizes what the government is spending and taking in. The latest version, released Monday, reports that Uncle Sam has paid $80 billion in net interest on the federal debt through the first four months of the 2011 fiscal year.
That represents a 9.1% increase over the same period last year.
The four-month period offers just a snapshot of the year. And the CBO report distills the gigantic federal budget, with its thousands of different programs, into nine big categories.
But make no mistake: The interest on the debt is a growing chunk of the budget. Last fiscal year, interest costs rose 13.2% -- more than any other category.
CBO, in its typically understated fashion, chalks up the current year's increase to "the substantial growth in the national debt over the past year."
The government has been forced to borrow a lot because of the persistently wide gap between spending and revenue.
This week alone, the Treasury Department plans to issue $191 billion in debt to investors at home and abroad. And in fiscal 2011, the federal government is expected to have its third consecutive annual deficit exceeding $1 trillion.
Wells Fargo's board of directors is under fire ahead of Tuesday's annual shareholder meeting amid criticism the board failed to do enough to prevent the fake account scandal. More
Reports Monday afternoon that Trump may propose a 15% corporate tax rate would seem to indicate that he won't lose sleep over adding substantially to the country's already-high debt. More
Larry Page's long-rumored "flying car" project is finally ready for takeoff. More
In 1998, Ntsiki Biyela won a scholarship to study wine making. Now she's about to launch her own brand. More
The Canadian province of Ontario is set to give thousands of dollars to low-income residents to see if its improves their health and job prospects. More