when was patient 71558 first prescribed a medication using the po route since 10/2103?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 71558 ) and prescriptions.route = 'po' and strftime('%y-%m',prescriptions.startdate) >= '2103-10' order by prescriptions.startdate asc limit 1