unable to DROP_EXTENDED_STATS

I am having performance issues with extended stats , I have a extension with below (NVL("X_REVERSAL_FLAG",'N'))

because of the function and quotes I am not able to delete stats .

exec DBMS_STATS.DROP_EXTENDED_STATS('OBIA_DW', 'W_AP_XACT_F', '(NVL("X_REVERSAL_FLAG",'N'))');

BEGIN DBMS_STATS.DROP_EXTENDED_STATS('OBIA_DW', 'W_AP_XACT_F', '(NVL("X_REVERSAL_FLAG",'N'))'); END;

                                                                                        *

ERROR at line 1:

ORA-06550: line 1, column 89:

PLS-00103: Encountered the symbol "))" when expecting one of the following:

) , * & = - + < / > at in is mod remainder not rem =>

<an exponent (**)> <> or != or ~= >= <= <> and or like like2