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: We undertook a descriptive analysis of Yellow Card records of 407 HIV - positive persons taking tenofovir disoproxil fumarate ( TDF ) as part of their antiretroviral therapy regimen and submitted to the Medicines and Healthcare Products Regulatory Agency ( MHRA ) with suspected kidney adverse effects .
### Output Text:
We undertook a descriptive analysis of Yellow Card records of 407 HIV - positive persons taking <span class="chemical">tenofovir disoproxil fumarate</span> ( <span class="chemical">TDF</span> ) as part of their antiretroviral therapy regimen and submitted to the Medicines and Healthcare Products Regulatory Agency ( MHRA ) with suspected kidney adverse effects . 