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: We report an undiagnosed case of myotonia congenita in a 24 - year - old previously healthy primigravida , who developed life threatening masseter spasm following a standard dose of intravenous suxamethonium for induction of anaesthesia .
### Output Text:
We report an undiagnosed case of myotonia congenita in a 24 - year - old previously healthy primigravida , who developed life threatening masseter spasm following a standard dose of intravenous <span class="chemical">suxamethonium</span> for induction of anaesthesia . 