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: Isotretinoin is contraindicated in pregnancy because of the many reported congenital abnormalities after maternal use ( including microphthalmos , orbital hypertelorism , and optic nerve hypoplasia ) .
### Output Text:
<span class="chemical">Isotretinoin</span> is contraindicated in pregnancy because of the many reported congenital abnormalities after maternal use ( including microphthalmos , orbital hypertelorism , and optic nerve hypoplasia ) . 