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: Images may need to be obtained using i . v . contrast material to enable diagnosis of ureteric stones or obstruction in patients with HIV infection who receive indinavir therapy .
### Output Text:
Images may need to be obtained using i . v . contrast material to enable diagnosis of ureteric stones or obstruction in patients with HIV infection who receive <span class="chemical">indinavir</span> therapy . 