Professional and Linguistic Assessments Board (PLAB) Practice Exam

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A positive result from a urinary antigen test would suggest infection with which organism?

  1. Legionella pneumophila

  2. Streptococcus pneumoniae

  3. Escherichia coli

  4. Mycoplasma pneumoniae

The correct answer is: Legionella pneumophila

A positive result from a urinary antigen test indicating an infection is most commonly associated with Legionella pneumophila. This organism is known for causing Legionnaires' disease, a serious form of pneumonia. The urinary antigen test specifically detects the presence of Legionella antigens in the urine, making it a useful diagnostic tool for quickly identifying this pathogen, particularly in patients presenting with pneumonia. While other organisms listed might be associated with respiratory infections, they do not have specific urinary antigen tests that are widely used or as reliable as that for Legionella. For instance, Streptococcus pneumoniae is typically diagnosed through sputum culture or blood tests rather than urinary antigen testing. Similarly, Escherichia coli is primarily linked with urinary tract infections, and diagnostic tests focus on urine culture. Mycoplasma pneumoniae, known to cause atypical pneumonia, also lacks a dedicated urinary antigen test, with diagnosis usually relying on serological methods or PCR. Thus, Legionella pneumophila stands out as the organism best indicated by a positive urinary antigen test.