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: Uni - and multivariate analyses were used to test the influence of the clinical variables : age , sex , stroke , myocardiopathy ( MP ) , duration of the test , mitral regurgitation ( MR ) and the MZ dose .
### Output Text:
Uni - and multivariate analyses were used to test the influence of the clinical variables : age , sex , stroke , myocardiopathy ( MP ) , duration of the test , mitral regurgitation ( MR ) and the <span class="chemical">MZ</span> dose . 