A query with a calculated field can do this like:
SELECT tbl_visit.ID, tbl_visit.admit, tbl_visit.disch, tbl_visit.pt_ID, IIf(DateDiff("d",[admit],DMax("disch","tbl_visit","admit <" & "#" & Format([admit],"mm\/dd\/yyyy") & "#"))<7,"y","n") AS re_admit_status
FROM tbl_visit
ORDER BY...