Question

A known case of AIDS with productive cough and fever was found to have consolidation in the right infrascapular area. X-Ray chest showed right lower lobe consolidation with CD4 count of 55 per microlitre. What is the most common cause of this?

• A. Staphylococcus aureus
• B. Streptococcus pneumoniae
• C. Mycoplasma
• D. Pneumocystis jirovecii

Hint:

HIV patients are highly susceptible to encapsulated bacteria; think about which organism most commonly causes lobar pneumonia.

Answer 1

The Correct Option is B

Step 1: Analyse the clinical scenario. The patient is a known HIV-positive individual with a CD4 count of 55 cells/microlitre (severely immunocompromised, normal >500), presenting with productive cough, fever, and right lower lobe consolidation on chest X-ray.

Step 2: Distinguish between opportunistic and common bacterial pathogens.

• Pneumocystis jirovecii pneumonia (PCP) is a common AIDS-defining illness but classically presents with bilateral interstitial (not lobar) infiltrates, dry cough, and CD4 typically <200. Lobar consolidation is not its hallmark.
• Streptococcus pneumoniae is the most common cause of bacterial pneumonia in HIV-positive patients at ALL CD4 counts. HIV-1 seropositive individuals have defective humoral immunity and are particularly susceptible to encapsulated bacteria like pneumococcus.
• Staphylococcus aureus and Mycoplasma are less common causes of pneumonia in this setting.

Step 3: Apply the CD4 count. At CD4 of 55, the patient is susceptible to all opportunistic infections; however, the presentation of lobar consolidation with productive cough most closely matches bacterial (pneumococcal) pneumonia rather than PCP.

Step 4: Conclusion. Streptococcus pneumoniae is the most common bacterial cause of pneumonia in HIV-infected patients and best explains the lobar consolidation seen here.