what is patient 3939's maximum alveolar-arterial gradient value?
select max(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3939 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'alveolar-arterial gradient' )