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: Sulfonamides and nitrofurantoins were associated with several birth defects , indicating a need for additional scrutiny .
### Output Text:
<span class="chemical">Sulfonamides</span> and <span class="chemical">nitrofurantoins</span> were associated with several birth defects , indicating a need for additional scrutiny . 