when has patient 94293 received a prescription for magnesium sulfate replacement (oncology) last until 01/2105?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 94293 ) and prescriptions.drug = 'magnesium sulfate replacement (oncology)' and strftime('%y-%m',prescriptions.startdate) <= '2105-01' order by prescriptions.startdate desc limit 1