As politicians begin to consider cuts to federal entitlements, one item appears to be totally off-limits. That's the Medicare Part D prescription drug benefit, the most popular new entity created since 1965, and also the most costly.

Through Part D premiums, seniors pay 25 percent of the total cost of Part D (about $1,800 per participant per year), with virtually all of the rest coming from automatic annual Treasury appropriations. Over the next decade, the Medicare trustees expect Part D spending to increase by 9.5 percent per year, and the total pay-as-you-go cost to the U.S. Treasury, including interest, will be about $1 trillion.

Of course, Part D has been fantastic for seniors and pharmaceutical companies. The average weighted monthly Part D premium has increased gradually from $26 in 2006 to about $39 in 2014. (High-income seniors now pay more.)