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: Increased mental slowing associated with the APOE epsilon4 allele after trihexyphenidyl oral anticholinergic challenge in healthy elderly .
### Output Text:
Increased mental slowing associated with the APOE epsilon4 allele after <span class="chemical">trihexyphenidyl</span> oral anticholinergic challenge in healthy elderly . 