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{{WBRQuestion
|QuestionAuthor=Gerald Chi
|ExamType=USMLE Step 1
|MainCategory=Microbiology, Pharmacology
|SubCategory=Pulmonology
|MainCategory=Microbiology, Pharmacology
|SubCategory=Pulmonology
|MainCategory=Microbiology, Pharmacology
|SubCategory=Pulmonology
|MainCategory=Microbiology, Pharmacology
|MainCategory=Microbiology, Pharmacology
|SubCategory=Pulmonology
|MainCategory=Microbiology, Pharmacology
|SubCategory=Pulmonology
|MainCategory=Microbiology, Pharmacology
|SubCategory=Pulmonology
|MainCategory=Microbiology, Pharmacology
|SubCategory=Pulmonology
|MainCategory=Microbiology, Pharmacology
|MainCategory=Microbiology, Pharmacology
|SubCategory=Pulmonology
|Prompt=A 32-year-old homosexual intravenous drug user is admitted with a worsening respiratory distress accompanied by fever and nonproductive cough. Arterial blood gas values are pH 7.52, PaCO2 28 mm Hg, HCO3 22 mEq/L, and PaO2 70 mm Hg when breathing room air. His CD4+ count is 150 cells per microliter. Chest X-ray reveals bilateral perihilar interstitial infiltrates suggesting an infection etiology. The causative organism is detected in bronchoalveolar lavage with silver stain. Two days after therapy, he starts to have dizziness, headache, coldness in hands and feet, pale skin, and chest pain. Peripheral blood smear shows irregularly fragmented erythrocytes. Supravital stain of the smear shows immature red cells with dark blue dots and curved linear structures in the cytoplasm. Which of the following medications is most likely to be the cause of his symptoms?

|Explanation=Pneumocystis pneumonia (PCP) is an opportunistic infection caused by Pneumocystis jirovecii. The risk of PCP increases when CD4+ cell levels are less than 200 cells/μl. Symptoms include fever, non-productive cough, shortness of breath, weight loss, and night sweats. Chest films typically show diffuse, symmetrical, perihilar interstitial infiltration that may progress to a homogenous, ground-glass opacification of lung fields. 


Hypoxemia, the most characteristic laboratory abnormality, can range from mild (room air arterial oxygen ≥70 mm Hg or alveolar-arterial O2 difference <35 mm Hg) to moderate (A-a DO2 ≥35 and <45 mm Hg) to severe (A-a DO2 ≥45 mm Hg).


TMP-SMX is the treatment of choice for PCP. For mild-to-moderate disease, alternative therapeutic regimens include: dapsone plus TMP, primaquine plus clindamycin, and atovaquone. For moderate-to-severe disease, clindamycin-primaquine or pentamidine can be used. Patients with moderate-to-severe disease should receive adjunctive corticosteroids as early as possible and certainly within 72 hours after starting specific PCP therapy. 


His hospital course is complicated by hemolytic anemia due to increased oxidative stress, which typically occurs in patients with glucose-6-phosphate dehydrogenase deficiency. Heinz bodies, bite cells, spherocytes, and reticulocytes may be evident on peripheral blood smear.

|AnswerA=Atovaquone

|AnswerAExp='''Incorrect'''<BR>For mild-to-moderate PCP, alternative therapeutic regimens include: dapsone plus TMP, primaquine plus clindamycin, and atovaquone. However, atovaquone generally does not cause hemolytic anemia.

|AnswerB=Clindamycin
|AnswerBExp='''Incorrect'''<BR>For mild-to-moderate disease, alternative therapeutic regimens include: dapsone plus TMP, primaquine plus clindamycin, and atovaquone. However, clindamycin generally does not cause hemolytic anemia.
|AnswerC=Methylprednisolone
|AnswerCExp='''Incorrect'''<BR>Patients with moderate-to-severe disease should receive adjunctive corticosteroids as early as possible and certainly within 72 hours after starting specific PCP therapy. However, methylprednisolone generally does not cause hemolytic anemia.

|AnswerD=Pentamidine

|AnswerDExp='''Incorrect'''<BR>For moderate-to-severe disease, clindamycin-primaquine or pentamidine can be used. However, pentamidine generally does not cause hemolytic anemia.

|AnswerE=Primaquine

|AnswerEExp='''Correct'''<BR>For mild-to-moderate disease, alternative therapeutic regimens include: dapsone plus TMP, primaquine plus clindamycin, and atovaquone. Common triggers of hemolytic anemia include sulfonamides and other drugs such as chloroquine, isoniazid, nalidixic acid, nitrofurantoin, and primaquine.

|RightAnswer=E
|Approved=No
}}