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: Primary dysmenorrhea is a syndrome characterized by painful uterine contractility caused by a hypersecretion of endometrial prostaglandins ; non - steroidal anti - inflammatory drugs are the first choice for its treatment .
### Output Text:
Primary dysmenorrhea is a syndrome characterized by painful uterine contractility caused by a hypersecretion of endometrial <span class="chemical">prostaglandins</span> ; non - steroidal anti - inflammatory drugs are the first choice for its treatment . 