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: Low - molecular - weight heparin ( LMWH ) offers theoretical advantages over UH , but is not currently considered in clinical guidelines as an alternative to UH in patients with prosthetic valves .
### Output Text:
<span class="chemical">Low - molecular - weight heparin</span> ( <span class="chemical">LMWH</span> ) offers theoretical advantages over <span class="chemical">UH</span> , but is not currently considered in clinical guidelines as an alternative to <span class="chemical">UH</span> in patients with prosthetic valves . 