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: High trough TAC level was the predominant factor that contributed to seizures in the early post - operative period after pediatric LT .
### Output Text:
High trough <span class="chemical">TAC</span> level was the predominant factor that contributed to seizures in the early post - operative period after pediatric LT . 