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: In the 5 - wk group , MIBG accumulation in the right and left ventricular wall was 35 % and 27 % of that in controls , respectively ( p less than 0 . 001 ) .
### Output Text:
In the 5 - wk group , <span class="chemical">MIBG</span> accumulation in the right and left ventricular wall was 35 % and 27 % of that in controls , respectively ( p less than 0 . 001 ) . 