instruction: for the last time, how much potassium chloride was prescribed during this month to patient 15986?
response: SELECT prescriptions.dose_val_rx FROM prescriptions WHERE prescriptions.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 15986) AND prescriptions.drug = 'potassium chloride' AND DATETIME(prescriptions.startdate, 'start of month') = DATETIME(CURRENT_TIME(), 'start of month', '-0 month') ORDER BY prescriptions.startdate DESC LIMIT 1