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: BACKGROUND : Recent evidence suggests that transient neurologic symptoms ( TNSs ) frequently follow lidocaine spinal anesthesia but are infrequent with bupivacaine .
### Output Text:
BACKGROUND : Recent evidence suggests that transient neurologic symptoms ( TNSs ) frequently follow <span class="chemical">lidocaine</span> spinal anesthesia but are infrequent with <span class="chemical">bupivacaine</span> . 