An organism isolated from a blood culture is a Gram-negative rod, urease and PD positive, with motility that is either positive or negative. Which organism does this describe?

The organism described fits well with Morganella morganii due to its key characteristics, including being a Gram-negative rod that is both urease-positive and phenylalanine deaminase (PD) positive.

Morganella morganii is known for its ability to hydrolyze urea, which leads to urease positivity and is significant in identifying the organism in clinical settings. The phenylalanine deaminase test is also positive for this organism, which further differentiates it from others in the same family. Moreover, the motility can vary, which aligns with the description in the question.

While other organisms in the choices may share some characteristics, such as Proteus species being urease positive, they differ in either the phenylalanine deaminase results or other biochemical markers. These differences are crucial for accurately identifying the organism. Thus, the attributes of the isolate align precisely with Morganella morganii, making it the correct answer.