Step 1

Pathology 12

An 18-year-old man presents to the clinic with a complaint of fatigue and a cough productive of green sputum for the past two days. He has no significant medical conditions. Social history reveals that he is sexually active with other men. Vital signs show a temperature of 101°F (38.3°C), heart rate 96 beats/minute, respirations 21/minute, and oxygen saturation 98% on room air. Physical examination reveals tachycardia, dyspnea, and bilateral crackles on auscultation of the lungs. Blood tests show increased levels of lactate dehydrogenase and a CD4 count 182/μL. HIV test is ordered but is pending. Chest x-ray reveals diffuse, bilateral, interstitial infiltrates. He is prescribed trimethoprim-sulfamethoxazole.

Which of the following is the most likely causative organism?

  • A) Haemophilus influenzae
  • B) Pneumocystis jirovecii
  • C) Pseudomonas aeruginosa
  • D) Staphylococcus aureus
  • E) Streptococcus pyogenes


Robert Li


Dr. Ted O'Connell