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: Provocation of postural hypotension by nitroglycerin in diabetic autonomic neuropathy ?
### Output Text:
Provocation of postural hypotension by <span class="chemical">nitroglycerin</span> in diabetic autonomic neuropathy ? 