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: An 82 - year - old man with treatment - resistant depression and early Alzheimer ' s disease was started on methylphenidate .
### Output Text:
An 82 - year - old man with treatment - resistant depression and early Alzheimer ' s disease was started on <span class="chemical">methylphenidate</span> . 