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: Our data suggest that there is an association between panic attacks , no matter if associated with PD or MDP , and hyperreactivity to an oral caffeine challenge test .
### Output Text:
Our data suggest that there is an association between panic attacks , no matter if associated with PD or MDP , and hyperreactivity to an oral <span class="chemical">caffeine</span> challenge test . 