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: At the beginning of nephrotic stage ( day 6 ) the concentration of plasma albumin dropped to a very low level , while that of apolipoprotein A - I increased abruptly ( 4 - fold increase ) and continued to rise , although less steeply , in the following days .
### Output Text:
At the beginning of nephrotic stage ( day 6 ) the concentration of plasma albumin dropped to a very low level , while that of apolipoprotein A - I increased abruptly ( 4 - fold increase ) and continued to rise , although less steeply , in the following days . 