CREDIT AGREEMENT among USMD HOLDINGS, INC., USMD INC., UROLOGY ASSOCIATES OF NORTH TEXAS, P.L.L.C., IMPEL MANAGEMENT SERVICES, L.L.C., IMPEL CONSULTING EXPERTS, L.L.C., MAT-RX DEVELOPMENT, L.L.C., USMD OF ARLINGTON GP, L.L.C., US LITHOTRIPSY, L.P., USMD ...