The ratings agency Moody’s has raised the stakes as the US approaches its debt limit.The firm has put the world's biggest economy on review for a credit score downgrade if American politicians continue their standoff and fail to raise the $14.3 trillion borrowing cap.Other agencies have warned they may also act - but Moody's is the first to begin a formal review.Bronywn Curtis from HSBC explains what this would mean for the US.