tell me the medication that patient 3929 has been prescribed for the first time in 05/this year.
select prescriptions.drug from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3929 ) and datetime(prescriptions.startdate,'start of year') = datetime(current_time,'start of year','-0 year') and strftime('%m',prescriptions.startdate) = '05' order by prescriptions.startdate asc limit 1