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: Pain was not explained by other diseases causing foot pain , like reflex sympathetic dystrophy , polyneuropathy , Morton ' s neuralgia , gout , osteoporosis , avascular necrosis , intermittent claudication , orthopaedic foot deformities , stress fractures , and hyperparathyroidism .
### Output Text:
Pain was not explained by other diseases causing foot pain , like reflex sympathetic dystrophy , polyneuropathy , Morton ' s neuralgia , gout , osteoporosis , avascular necrosis , intermittent claudication , orthopaedic foot deformities , stress fractures , and hyperparathyroidism . 