On Thursday, shares of the nation’s biggest insurer of financial risk, MBIA, fell 26 percent... The move came a day after Standard & Poor’s downgraded another bond insurer and assigned a negative outlook to four companies, including MBIA. MBIA’s disclosure, which came in a data table posted on its Web site late Wednesday without any explanation, highlights the crisis in confidence that has racked the credit market. Investors, specialists say, are scared of what they do not know and no longer trust what they are told by the professionals who are supposed to know. * * * MBIA, whose name...