An organism that is positive with a modified acid-fast stain and recovered from a sputum specimen is likely which of the following?

The modified acid-fast stain is used to detect specific types of bacteria that share characteristics with Mycobacterium species, particularly those that have mycolic acids in their cell walls. Nocardia species, which are aerobic actinomycetes, exhibit this property and show up as positive on modified acid-fast stains. Their distinctive branching, filamentous morphology also aids in their identification.

In clinical practice, when an organism is isolated from a sputum specimen and tests positive with a modified acid-fast stain, Nocardia is a strong consideration, especially in the context of respiratory infections. This organism can cause opportunistic infections, particularly in immunocompromised hosts.

Other options like Bacillus, Corynebacterium, and Listeria do not possess the same acid-fast properties, making them unlikely candidates for positive results with a modified acid-fast stain. Bacillus species may cause various infections but lack the characteristic staining; Corynebacterium species typically show irregular staining patterns and do not possess the mycolic acids that confer acid-fastness; Listeria species, while pathogenic and often encountered in foodborne illnesses, do not exhibit modified acid-fast reactions.

Thus, given the specific characteristics of Nocardia, along with its relevance in respiratory specimens,