when was patient 3929 prescribed a medication during their current hospital visit for the last time via the po/ng route?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3929 and admissions.dischtime is null ) and prescriptions.route = 'po/ng' order by prescriptions.startdate desc limit 1