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 3 patients ( age 37 - 52 years ) with acute respiratory insufficiency who developed prolonged weakness following the discontinuation of ND - NMBAs .
### Output Text:
We report 3 patients ( age 37 - 52 years ) with acute respiratory insufficiency who developed prolonged weakness following the discontinuation of <span class="chemical">ND - NMBAs</span> . 