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: Our results provide further support for the hypothesis of an NMDA receptor dysfunction in the pathophysiology of schizophrenia .
### Output Text:
Our results provide further support for the hypothesis of an <span class="chemical">NMDA</span> receptor dysfunction in the pathophysiology of schizophrenia . 