Final answer:
The non-spore forming, branching Gram-positive bacillus causing infection in an immunocompromised patient is most likely Nocardia spp., as it fits the clinical description and has a pattern of causing pulmonary infections in such individuals.
Step-by-step explanation:
A non-spore forming, branching, Gram-positive (GP) bacillus that was recovered from an immunocompromised patient with shortness of breath and wheezing likely fits the clinical picture of an infection caused by Nocardia spp. This genus of bacteria can cause pulmonary infections, especially in immunocompromised individuals, and matches the description given - non-spore forming and branching rods. Among the available options, Corynebacterium spp. and Erysipelothrix do not typically present with branching rods, and while Listeria monocytogenes is a GP bacillus, it does not form branches.
It's important to note that while the reference information provided lists other genera of GP bacteria such as Bacillus and Mycoplasma, these do not match the clinical picture described. Bacillus spp. are endospore-forming, and Mycoplasma lacks a cell wall and does not appear Gram-positive under microscopic examination.