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: We randomly selected 29 patients with PD , 27 with MDP , 25 with major depression without panic attacks ( MD ) , and 28 healthy volunteers .
### Output Text:
We randomly selected 29 patients with PD , 27 with MDP , 25 with major depression without panic attacks ( MD ) , and 28 healthy volunteers . 