Question: Why did Mrs. Annabelle Pouros take leucovorin 100 mg injection on 1970/12/19? State the reason.
SQL: SELECT T2.reasondescription FROM patients AS T1 INNER JOIN medications AS T2 ON T1.patient = T2.PATIENT WHERE T1.prefix = 'Mrs.' AND T1.first = 'Annabelle' AND T1.last = 'Pouros' AND T2.start = '1970-12-19' AND T2.description = 'Leucovorin 100 MG Injection'