Which is the most appropriate nonselective medium for the recovery of mycobacteria from a heavily contaminated specimen?

The most appropriate nonselective medium for the recovery of mycobacteria from a heavily contaminated specimen is Petragnani's agar. This agar is specifically formulated to support the growth of Mycobacterium tuberculosis and other mycobacteria while providing a relatively conducive environment for their recovery.

Petragnani's agar utilizes a combination of nutrients and a specific pH to favor the growth of mycobacteria over most other bacteria. It also contains inhibitors that help mitigate the effects of contaminants that could overshadow the growth of mycobacteria, making it particularly useful in samples that are heavily contaminated.

Other media like Lowenstein-Jensen agar and Middlebrook 7H10 agar, while they are widely used for mycobacterial culture, are considered either selective or more specific, which may not be as effective in cases where there is significant contamination from other microorganisms. The American Thoracic Society medium, although designed for respiratory pathogens, may not provide the same level of nonselectivity as Petragnani's agar in heavily contaminated environments. Thus, Petragnani's agar stands out as the most suitable option for this scenario.