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: CONCLUSION : The epsilon4 allele in healthy elderly was associated with increased subjective mental slowing after trihexyphenidyl anticholinergic challenge .
### Output Text:
CONCLUSION : The epsilon4 allele in healthy elderly was associated with increased subjective mental slowing after <span class="chemical">trihexyphenidyl</span> anticholinergic challenge . 