how much metronidazole is prescribed to patient 5905 last time until 48 months ago?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 5905 ) and prescriptions.drug = 'metronidazole' and datetime(prescriptions.startdate) <= datetime(current_time,'-48 month') order by prescriptions.startdate desc limit 1