what was the last time patient 2425 received a prescription for thiamine, since 04/2101?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 2425 ) and prescriptions.drug = 'thiamine' and strftime('%y-%m',prescriptions.startdate) >= '2101-04' order by prescriptions.startdate desc limit 1