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: MATERIALS AND METHODS : We investigated the causes of thrombotic microangiopathy during a 1 - year period , from June 2003 to June 2004 , at the King Fahad National Guard Hospital in Riyadh , Saudi Arabia , by reviewing the slides of all transplant biopsies ( n = 25 ) performed during this interval .
### Output Text:
MATERIALS AND METHODS : We investigated the causes of thrombotic microangiopathy during a 1 - year period , from June 2003 to June 2004 , at the King Fahad National Guard Hospital in Riyadh , Saudi Arabia , by reviewing the slides of all transplant biopsies ( n = 25 ) performed during this interval . 