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: The occurrence of nephrotoxicity by means of an increase in serum creatinine and evaluation of efficacy via amikacin serum concentrations with respective pathogens were assessed .
### Output Text:
The occurrence of nephrotoxicity by means of an increase in serum <span class="chemical">creatinine</span> and evaluation of efficacy via <span class="chemical">amikacin</span> serum concentrations with respective pathogens were assessed . 