In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: We report a case of 54 - year - old woman with medical history of mitral valve prolapse and migraines , who was admitted to the hospital for substernal chest pain and electrocardiogram demonstrated 1 / 2 mm ST - segment elevation in leads II , III , aVF , V5 , and V6 and positive troponin I .
### Output Text:
We report a case of 54 - year - old woman with medical history of mitral valve prolapse and migraines , who was admitted to the hospital for substernal chest pain and electrocardiogram demonstrated 1 / 2 mm ST - segment elevation in leads II , III , aVF , V5 , and V6 and positive troponin I . 