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: CONCLUSION : Ureteric obstruction caused by precipitated indinavir crystals may be difficult to diagnose with unenhanced CT .
### Output Text:
CONCLUSION : Ureteric obstruction caused by precipitated <span class="chemical">indinavir</span> crystals may be difficult to diagnose with unenhanced CT . 