WBR0253

Author	[[PageAuthor::Ogheneochuko Ajari, MB.BS, MS [1] (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Pulmonology, SubCategory::Infectious Disease
Prompt	[[Prompt::A 46-year-old homeless man presents to the emergency department (ED) with complaints of fever, dyspnea, and non-productive cough. He explains that his symptoms did not occur acutely, but have progressively worsened over the past few weeks. The patient does not smoke, but drinks 6-8 beers every day. He lives in a shelter for the homeless. His past medical history is significant for end-stage renal disease and kidney transplantation, for which he currently receives immunosuppressive therapy. In the ED, the patient's temperature is 39.7 °C (103.5 °F), blood pressure is 138/86 mmHg, and heart rate is 102/min. He appears very thin and severely malnourished. Physical examination is remarkable for coarse crackles on anterior auscultation of the right upper lung field. Chest x-ray demonstrates nodular infiltrates and cavitations in the right upper lobe. The patient is admitted and is started on broad-spectrum antimicrobial therapy. The next day, sputum culture shows an aerobic, gram-positive rod with a beaded acid-fast appearance on microscopy. Which antimicrobial agent is the optimal monotherapeutic option to treat this patient's condition?]]
Answer A	AnswerA::Sulfisoxazole
Answer A Explanation	[[AnswerAExp::Sulfonamides, such as sulfadiazine and sulfisoxazole, are the first line antimicrobial agents to treat nocardiosis.]]
Answer B	AnswerB::Trimethoprim
Answer B Explanation	[[AnswerBExp::Trimethoprim alone is insufficient to treat nocardiosis. However, TMP-SMX, which includes sulfonamide, is a possible pharmacological therapy against Nocardia.]]
Answer C	AnswerC::Rifampin
Answer C Explanation	[[AnswerCExp::The combinatoin of rifampin, isoniazid, ethambutol, and pyrazinamide is administered to patients with active tuberculosis (TB) caused by Mycobacterium tuberculosis. TB should always be in the differential diagnosis of nocardiosis, since it manifests similarly and has similar risk factors. However, this patient's sputum culture reveals aerobic, gram-positive rods. Unlike TB, the acid-fast appearance in Nocardia is often described as "beaded".]]
Answer D	AnswerD::Penicillin
Answer D Explanation	[[AnswerDExp::Penicillin is not an effective therapy to treat nocardiosis. In contrast, penicillin is the first line agent against Actinomyces, which are anaerobic, gram-positive rods with branching filaments. Actinomyces should always be in the differential diagnosis of Nocardia. Unlike Nocardia, Actinomyces is anaerobic and is not acid-fast.]]
Answer E	AnswerE::Rifabutin
Answer E Explanation	[[AnswerEExp::Rifamycins (rifampin and rifabutin) are not effective to treat nocardiosis. Rifampin is among the combination therapy to treat tuberculosis. Rifabutin is a more expensive rifamycin that may also be used to treat tuberculosis, but it is usually reserved for prophylaxis against disseminated Mycobacterium avium intracellulare among HIV-positive patients.]]
Right Answer	RightAnswer::A
Explanation	[[Explanation::Nocardiosis is an opportunistic infection that is caused by Nocardia asteroides, an aerobic, acid-fast, gram-positive rod with branching filaments. Although N. asteroides is also variably acid fast, it may be distinguished from Mycobacterium species by its "beaded" acid-fast appearance on microscopy. Although Actinomyces also appears as a gram-positive rod with branching filaments, it is anaerobic and is not acid-fast. Nocardia commonly infects immunocompromised patients, such as patients with malignancies, HIV-positive patients, and individuals on immunosuppressive therapy. However, N. asteroides may colonize immunocompetent individuals and patients with structural lung disease (cystic fibrosis and bronchiectasis) without causing a pulmonary infection. N. asteroides is usually transmitted by inhalation. Manifestations in the human host appear subacutely, similarly to infections with Mycobacterium tuberculosis; they include high-grade fever, non-productive cough, dyspnea, hemoptysis, and constitutional symptoms such as weight loss, fatigue, and night sweats. Chest x-ray typically demonstrates nodular or consolidation infiltrates along with cavitary lesions and/or parapneumonic pleural effusions. The organism spreads contiguously to the pericardium and mediastinum or hematogenously to the CNS (cerebral nocardiosis) and causes extrapulmonary manifestations, such as abscess formation and chronic granulomas. Sulfonamides, such as sulfadiazine and sulfisoxazole, are the first line antimicrobial agents to treat nocardiosis. Prior to confirmation with susceptibility, empirical combination therapy that includes sulfonamides (e.g. TMP-SMX and imipenem/ceftriaxone) is frequently administered to patient with nocardiosis. Educational Objective: Nocardiosis is an opportunistic infection that is caused by Nocardia asteroides, an aerobic, acid-fast, gram-positive rod with branching filaments. Sulfonamides, such as sulfadiazine and sulfisoxazole, are the first line antimicrobial agents to treat nocardiosis. References: Wilson JW. Nocardiosis: updates and clinical overview. Mayo Clin Proc. 2012;87(4):403-7. First Aid 2014 page 134]]
Approved	Approved::Yes
Keyword	WBRKeyword::Nocardia, WBRKeyword::Nocardiosis, WBRKeyword::Sulfonamides, WBRKeyword::Sulfisoxazole, WBRKeyword::Sulfadiazine, WBRKeyword::TMP-SMX, WBRKeyword::Fever, WBRKeyword::Chest pain, WBRKeyword::Dyspnea, WBRKeyword::Gram-positive, WBRKeyword::Antibiotics, WBRKeyword::Branching filaments, WBRKeyword::Rods, WBRKeyword::Acid-fast
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