WBR0050
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| Author | [[PageAuthor::William J Gibson (Reviewed by Rim Halaby, M.D. [1] and Yazan Daaboul)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Microbiology |
| Sub Category | SubCategory::General Principles, SubCategory::Infectious Disease |
| Prompt | [[Prompt::A 52-year-old homeless man presents to the emergency department after coughing up blood. He has experienced increasing cough over the past 2 weeks after recovering from an upper respiratory tract infection. Upon further questioning, the patient reports he smokes one and a half packs of cigarettes and drinks 8 beers daily for the past 20 years. The patient also states he has unintentionally lost 10 kilograms of weight in the past 6 months. His blood pressure is 138/88 mmHg, heart rate is 98/min, and temperature is 101.5 °F (38.6 °C). Physical examination is remarkable for crackles heard over the right chest. In the ED, a chest xray is ordered; it reveals a right upper lobe opacity. Which of the following media may be used to isolate the organism most likely responsible for this patient's condition?]] |
| Answer A | AnswerA::Eaton's agar |
| Answer A Explanation | [[AnswerAExp::Eaton’s agar can be used to culture Mycoplasma pneumoniae.]] |
| Answer B | AnswerB::Charcoal yeast extract |
| Answer B Explanation | [[AnswerBExp::Charcoal yeast extract can be used to culture Legionella pneumophila.]] |
| Answer C | AnswerC::MacConkey’s agar (MAC) |
| Answer C Explanation | [[AnswerCExp::MacConkey’s agar (MAC) is used to culture a variety of gram-negative organisms and cannot be used to culture Mycobacterium tuberculosis.]] |
| Answer D | AnswerD::Tellurite agar |
| Answer D Explanation | [[AnswerDExp::Tellurite agar can be use to culture Corynebacterium diptheriae.]] |
| Answer E | AnswerE::Lowenstein-Jensen agar |
| Answer E Explanation | [[AnswerEExp::Lowenstein-Jensen agar can be used to culture Mycobacterium tuberculosis, the organism most likely responsible for this patient's condition.]] |
| Right Answer | RightAnswer::E |
| Explanation | [[Explanation::The patient is most likely infected with M. tuberculosis (TB). He has several risk factors for TB, including homelessness and alcoholism. Other infectious organisms, such as Klebsiella pneumoniae, are also associated with pneumonia in alcoholics, typically due to aspiration of intestinal flora. However, these infections typically cause focal lower lobe consolidation. Primary TB infection generally follows a subacute clinical course characterized by the development of fever, cough, night sweats, and weight loss. A ghohn complex, formed of a ghohn focus in the lower lobes and hilar/perihilar lymphadenopathy, are classically observed in primary TB infections. Following primary infection, the organism is engulfed by macrophages for phagocytosis. However, TB has the capacity to proliferate intracellularly. An immune response is mounted using Th1 immune system and activation of macrophage. This response results in the development of a granuloma and subsequent formation of a caseous (coagulative) necrosis.
Conversely, reactivation of M. tuberculosis demonstrates an opacity that is usually present in the highly oxygenated upper lobes of the lungs and causes a fibrocaseous cavitary lesion. M. tuberculosis may be cultured on Lewenstein-Jensen media, but the culture process requires approximately 4 weeks due to the slow doubling time of Mycobacterium tuberculosis. Accordingly, an acid-fast bacilli stain may be more helpful for the diagnosis of TB. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Tuberculosis, WBRKeyword::TB, WBRKeyword::Microbiology, WBRKeyword::Bacteria, WBRKeyword::Laboratory, WBRKeyword::Alcoholism, WBRKeyword::Pulmonary, WBRKeyword::Lowenstein-jensen, WBRKeyword::agar, WBRKeyword::ghohn, WBRKeyword::complex, WBRKeyword::hilar, WBRKeyword::adeonopathy |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |