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 possible aetiologies of valproate - induced encephalopathy and NMDA receptor - associated encephalitis present a diagnostic dilemma .
### Output Text:
The possible aetiologies of <span class="chemical">valproate</span> - induced encephalopathy and <span class="chemical">NMDA</span> receptor - associated encephalitis present a diagnostic dilemma . 