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 results show that , unlike PREGS , 11 - ketopregnenolone sulphate and epipregnanolone sulphate failed to block the effect of scopolamine , suggesting that altering the modulation of NMDA receptors diminishes the memory - enhancing effects of PREGS .
### Output Text:
The results show that , unlike <span class="chemical">PREGS</span> , <span class="chemical">11 - ketopregnenolone sulphate</span> and <span class="chemical">epipregnanolone sulphate</span> failed to block the effect of <span class="chemical">scopolamine</span> , suggesting that altering the modulation of <span class="chemical">NMDA</span> receptors diminishes the memory - enhancing effects of <span class="chemical">PREGS</span> . 