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 long - term use of nondepolarizing neuromuscular blocking agents ( ND - NMBA ) has recently been implicated as a cause of prolonged muscle weakness , although the site of the lesion and the predisposing factors have been unclear .
### Output Text:
The long - term use of <span class="chemical">nondepolarizing neuromuscular blocking agents</span> ( <span class="chemical">ND - NMBA</span> ) has recently been implicated as a cause of prolonged muscle weakness , although the site of the lesion and the predisposing factors have been unclear . 