Question: what is a left bundle branch block in the heart
[{'answer': 'a cardiac conduction abnormality', 'provenance': [{'wikipedia_id': '3538531', 'title': 'Left bundle branch block', 'start_paragraph_id': 1, 'start_character': 35, 'end_paragraph_id': 1, 'end_character': 67, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '3538531', 'title': 'Left bundle branch block', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 256, 'bleu_score': 0.950578622812921, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9781737337219240087}, 'section': 'Section::::Abstract.'}]}]