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: One patient died , having presented in hepatic failure , and another , who had been taking methyldopa for 7 years , showed slower clinical and biochemical resolution over a period of several months .
### Output Text:
One patient died , having presented in hepatic failure , and another , who had been taking <span class="chemical">methyldopa</span> for 7 years , showed slower clinical and biochemical resolution over a period of several months . 