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: An investigation of the pattern of kidney injury in HIV - positive persons exposed to tenofovir disoproxil fumarate : an examination of a large population database ( MHRA database ) .
### Output Text:
An investigation of the pattern of kidney injury in HIV - positive persons exposed to <span class="chemical">tenofovir disoproxil fumarate</span> : an examination of a large population database ( MHRA database ) . 