when was for the last time during the last hospital encounter patient 31696 prescribed a medication via po/ng route?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 31696 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and prescriptions.route = 'po/ng' order by prescriptions.startdate desc limit 1