The Baylor College of Medicine has annually been considered within the top ten medical schools in the nation; likewise, the MD Anderson Cancer Center has consistently ranked as one of the top two U.S. hospitals specializing in cancer care by U.S. News & World Report since 1990. The Menninger Clinic, a renowned psychiatric treatment center, is affiliated with Baylor College of Medicine and The Methodist Hospital System. With hospital locations nationwide and headquarters in Houston, the Triumph Healthcare hospital system is the third largest long term acute care provider nationally.

Answer this question, if possible (if impossible, reply "unanswerable"):  What is the fourth largest acute care center in the U.S.?
unanswerable