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: We report a newborn infant with multiple congenital anomalies ( anotia and Taussig - Bing malformation ) due to exposure to isotretinoin within the first trimester .
### Output Text:
We report a newborn infant with multiple congenital anomalies ( anotia and Taussig - Bing malformation ) due to exposure to <span class="chemical">isotretinoin</span> within the first trimester . 