Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

Stocks ended broadly higher Monday, and the S&P 500 Index (SNPINDEX:^GSPC) advanced for a fifth straight day after China's August export numbers easily topped expectations. Only one in five U.S. stocks lost ground today, and the benchmark S&P 500 added 16 points, or 1%, ending at 1,671. But enough about today's upbeat market -- while impressive, it wasn't enough to save these three S&P issues. 

Tenet Healthcare (NYSE:THC) earned a distinction that earned no cheers from shareholders today. The stock ended up as one of the index's three worst performers for a second straight session, losing 3.7% Monday after Friday's 2% fallout. The hospital operator, which already has a dangerously high debt-to-equity ratio, is financing its acquisition of Vanguard Health Systems with $4.6 billion in new debt, all of which Moody's gave junk bond ratings today.

Oil refiner Marathon Petroleum (NYSE:MPC) shed 1.8% as the entire refinery industry was called out as a poor investment by investment bank Simmons & Company. Marathon in particular was called out by Simmons, which downgraded the stock to neutral from overweight in the face of rising oil prices. Rising prices often mean lower margins for refiners like Marathon, and with tensions in the Middle East causing supply concerns, markets have been putting a premium on crude. 

Lastly, shares of the Buffalo-based M&T Bank (NYSE:MTB) dropped 1.6% Monday, as the bank reportedly attempts to pawn off $1.4 billion of asset-backed securities in the bond market. The bonds in question are a collection of auto loans, and considering that the debt will be a bundle of loans to borrowers with high credit, the company should ideally be able to hawk them without much trouble. The only potential issue with the offering is that M&T isn't alone; creditors are rushing to pawn off asset-backed securities in the bond market, meaning M&T may have to offer higher rates to attract investors.

Fool contributor John Divine has no position in any stocks mentioned. You can follow him on Twitter, @divinebizkid, and on Motley Fool CAPS, @TMFDivine.

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