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: While the involvement of metronidazole in the aetiology of the haemolytic - uraemic syndrome is not established firmly , the action of this drug in sensitizing tissues to oxidation injury and the reported evidence of oxidation changes in the haemolytic - uraemic syndrome suggest a possible link between metronidazole treatment and some cases of the haemolytic - uraemic syndrome .
### Output Text:
While the involvement of <span class="chemical">metronidazole</span> in the aetiology of the haemolytic - uraemic syndrome is not established firmly , the action of this drug in sensitizing tissues to oxidation injury and the reported evidence of oxidation changes in the haemolytic - uraemic syndrome suggest a possible link between <span class="chemical">metronidazole</span> treatment and some cases of the haemolytic - uraemic syndrome . 