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 describe a 25 - year - old woman with pre - existing mitral valve prolapse who developed intractable ventricular fibrillation after consuming a " natural energy " guarana health drink containing a high concentration of caffeine .
### Output Text:
We describe a 25 - year - old woman with pre - existing mitral valve prolapse who developed intractable ventricular fibrillation after consuming a " natural energy " guarana health drink containing a high concentration of <span class="chemical">caffeine</span> . 