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 lead - treated group exhibited a rise in blood pressure and plasma MDA concentration , a fall in urinary NOx excretion , and a paradoxical rise in vascular and renal tissue eNOS and iNOS expression .
### Output Text:
The <span class="chemical">lead</span> - treated group exhibited a rise in blood pressure and plasma <span class="chemical">MDA</span> concentration , a fall in urinary NOx excretion , and a paradoxical rise in vascular and renal tissue eNOS and iNOS expression . 