AN A319 RESPONDED TO AN RA BY DESCENDING CLEAR OF TRAFFIC WHILE ON APPROACH TO IAD.

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ON CHK-IN WITH POTOMAC APCH; WE WERE TOLD THAT THE ARPT WAS TURNING AROUND AND TO EXPECT ILS RWY 19R. WE WERE GIVEN VECTORS AND A DSCNT AND WERE ON A BASE LEG ABOUT 15 NW OF IAD AT 4000 FT. WE NOTICED TFC ON TCAS LEVEL AT OUR ALT AND CLOSING FROM ABOUT 5 MI OUT. POTOMAC APCH CALLED US WITH A TFC RPT AND SAID 'VFR TFC 12 O'CLOCK; 2 MI AND...LET ME VERIFY THE ALT.' AT THE SAME TIME WE ACQUIRED THE TFC VISUALLY AT 12:30 O'CLOCK POS AND ABOUT 1.5 MI. I STARTED A HARD TURN TO THE L TO AVOID TFC JUST AS WE GOT A TCAS RA TO DSND AND SAW DSCNT OF 2100 FPM DOWN WAS REQUIRED. I COMPLIED WITH THE RA WHILE THE FO TOLD POTOMAC WE WERE TURNING AND DSNDING FOR THE RA. AT 3100 FT; WE GOT A CLR OF CONFLICT AND CLBED BACK UP TO 4000 FT AND RETURNED TO A 080 DEG HDG. SHORTLY AFTER WE WERE GIVEN A HDG OF 160 DEGS TO INTERCEPT THE RWY 19L FINAL APCH COURSE AND TO DSND TO 3000 FT. THIS MADE SENSE AS IT WAS THE LAST APCH WE WERE TOLD TO EXPECT. A COUPLE OF MI LATER WE WERE TOLD TO FLY 170 KTS TO FEMKO. THIS IS ON RWY 19R; SO I STOPPED THE TURN TO PARALLEL THE FINAL AND THE FO CONFIRMED THAT WE WERE SUPPOSED TO BE INTERCEPTING RWY 19L. THE CTLR SAID 'NO; CLRED THE ILS 19R.' WE INTERCEPTED RWY 19R AND LANDED WITHOUT FURTHER INCIDENT. RPTED THIS TO THE TWR AND WAS INSTRUCTED ON GND TO CALL THE SUPVR WHEN WE LANDED; WHICH WE DID.