instruction: how many times adefovir dipivoxil has been prescribed to patient 18159 on their last hospital encounter?
response: SELECT COUNT(*) FROM prescriptions WHERE prescriptions.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 18159 AND NOT admissions.dischtime IS NULL ORDER BY admissions.admittime DESC LIMIT 1) AND prescriptions.drug = 'adefovir dipivoxil'