A gram-negative bacillus recovered from a child's urine is oxidase negative, lactose negative, urease positive, and motile. The most likely identification is which species?

• A. Klebsiella pneumoniae
• B. Proteus mirabilis
• C. Pseudomonas aeruginosa
• D. Escherichia coli

Answer: (B)

Think about identifying a Gram-negative rod from urine by combining biochemical traits. Oxidase negative points away from Pseudomonas and toward Enterobacteriaceae. Lactose negative means it does not ferment lactose, so common enterics like E. coli and Klebsiella are unlikely. Urease positive is a hallmark of Proteus species, which also happen to be motile. Proteus mirabilis is known for strong urease activity and high motility (swarming on growth media). Putting these traits together—oxidase negative, lactose negative, urease positive, and motile—best fits Proteus mirabilis. The other options either ferment lactose, are oxidase positive, or are non-motile, which doesn’t align with the given profile.