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: A significant difference in MMSE was observed at T30 in patients who received 0 . 50 mg / kg of ketamine .
### Output Text:
A significant difference in MMSE was observed at T30 in patients who received 0 . 50 mg / kg of <span class="chemical">ketamine</span> . 