WBR0297
| Author | [[PageAuthor::Ogheneochuko Ajari, MB.BS, MS [1]]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Microbiology |
| Sub Category | SubCategory::Infectious Disease |
| Prompt | [[Prompt::A 40-year old male presents to the emergency room with complaints of headache, fever, vomiting and rash, which began from the ankles and wrists and started migrating towards the trunk. Physical examination reveals fever of 102F, maculopapular rash on the ankles and wrists with ankle and wrist swelling. What is the likely causative organism of these symptoms?]] |
| Answer A | AnswerA::Coxiella burnetti |
| Answer A Explanation | AnswerAExp::Incorrect. Coxiella burnetti causes Q fever and presents with NO rash. Weil-Felix test is also negative |
| Answer B | AnswerB::Rickettsia typhi |
| Answer B Explanation | AnswerBExp::Incorrect. Rickettsia typhi causes Endemic typhus transmitted by fleas and the rash typically begin from the trunk |
| Answer C | AnswerC::Rickettsia rickettsii |
| Answer C Explanation | AnswerCExp::Correct. See explanation |
| Answer D | AnswerD::Ehrlichia chaffeensis |
| Answer D Explanation | AnswerDExp::Incorrect. Ehrlichia chaffeensis is transmitted by ticks and does not present with a rash. It causes Ehrlichiosis and typically presents with thrombocytopenia and leukopenia |
| Answer E | AnswerE::Rickettsia prowazekii |
| Answer E Explanation | AnswerEExp::Incorrect. Rickettsia prowazekii causes Epidemic typhus and rash starts from the trunk |
| Right Answer | RightAnswer::C |
| Explanation | [[Explanation::The patient in this vignette has Rocky Mountain spotted fever caused by Rickettsia rickettsii (Choice C). It is an aerobic gram-negative bacillus. It is prevalent in the East Coast mountainous areas with a 2-12 days incubation period. It presents with a rash (maculopapular → petechiae) which starts from the ankles and wrists and then spreads to trunks, palms, soles and face (centripetal rash). Ankle and wrist swelling also occurs. The diagnosis of the disease may be confused with gastrointestinal symptoms, periorbital swelling, stiff neck, conjunctivitis and arthralgias.
Transmission is by the hard tick, Dermacentor, with small wild rodents and larger wild and domestic animals such as dogs being reservoirs. It invades endothelial cells lining capillaries, causing vasculitis in many organs including brain, liver, skin, lungs, kidney and gastrointestinal tract. Diagnosis is with serological Immunofluorescence Antibody test and a 4-fold increase in titer is diagnostic. The Weil-Felix test is positive although no longer used. NOTE: Palm and sole rash is also seen in Coxsackie virus A infection (hand, foot and mouth disease and Syphilis. (MNEUMONIC: CARS - Coxsackie virus A, Rocky Mountain spotted fever, Syphilis) Educational objective Rocky Mountain spotted fever is caused by Rickettsia rickettsii, with a characteristic maculopapular rash that begins from the ankles and wrist and migrating towards the trunks and other parts of the body References: First AID for the USMLE Step 1 Pg 155 |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Rash beginning from ankles and wrists |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |