WBR0050
| 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::Pulmonology, SubCategory::General Principles, SubCategory::Infectious Disease |
| Prompt | [[Prompt::A 52-year-old alcoholic and 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. His blood pressure is 138/88 mmHg, heart rate is 98/min, and temperature is 101.5°F (38.6°C). A chest xray shows a right upper lobe opacity. Which of the following media could be used to isolate the most likely causal organism?]] |
| 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 causal organism in this vignette.]] |
| Right Answer | RightAnswer::E |
| Explanation | [[Explanation::The patient is most likely suffering from M. tuberculosis (TB). He has several risk factors for TB including homelessness and alcoholism. Some other infections such as Klebsiella are also known for causing pneumonia in alcoholics, typically due to aspiration of intestinal flora. However, these infections typically cause focal lower lobe consolidation. Primary TB infection usually follows a subacute clinical course. A ghohn complex, formed of ghohn focus in the lower lobes and hilar/perihilar lymphadenopathy, is observed. After primary infection, the organisms is engulfed by macropahges for phagocytosis. However, TB has the capacity to proliferate intracellularly. An immune response is mounted using Th1 and further macrophage activation leading to the formation of a granuloma, with the eventual 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 lung, causing a fibrocaseous cavitary lesion. M. tuberculosis can be cultured on Lewenstein-Jensen media, but the culture process takes approximately 4 weeks due to the slow doubling time of mycobacterium tuberculosis. An acid-fast bacilli stain may thus be more helpful in 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:: |