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: Clinical and biochemical monitoring at regular intervals for evidence of hepatitis is advised during long term treatment with ketoconazole to prevent possible serious hepatic injury .
### Output Text:
Clinical and biochemical monitoring at regular intervals for evidence of hepatitis is advised during long term treatment with <span class="chemical">ketoconazole</span> to prevent possible serious hepatic injury . 