Tell me the reason for Lavelle Vandervort's encounter on 11/20/2013?
reason for encounter refers to encounters.REASONDESCRIPTION; on 11/20/2013 refers to encounters.DATE = '2013-11-20';
SELECT T2.REASONDESCRIPTION FROM patients AS T1 INNER JOIN encounters AS T2 ON T1.patient = T2.PATIENT WHERE T2.DATE = '2013-11-20' AND T1.first = 'Lavelle' AND T1.last = 'Vandervort'