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: OBJECTIVE : We describe the unique CT features of ureteric calculi in six HIV - infected patients receiving indinavir , the most commonly used HIV protease inhibitor , which is associated with an increased incidence of urolithiasis .
### Output Text:
OBJECTIVE : We describe the unique CT features of ureteric calculi in six HIV - infected patients receiving <span class="chemical">indinavir</span> , the most commonly used HIV protease inhibitor , which is associated with an increased incidence of urolithiasis . 