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: Follow - up visits were scheduled with 2 - drug therapy every 6 to 8 weeks , an average of 10 . 3 visits per patient , with 741 batteries of 6 liver function tests and 714 creatine phosphokinase levels measured .
### Output Text:
Follow - up visits were scheduled with 2 - drug therapy every 6 to 8 weeks , an average of 10 . 3 visits per patient , with 741 batteries of 6 liver function tests and 714 <span class="chemical">creatine</span> phosphokinase levels measured . 