WBR0057
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| Author | William J Gibson (Reviewed by Yazan Daaboul) |
|---|---|
| Exam Type | USMLE Step 1 |
| Main Category | Microbiology |
| Sub Category | Infectious Disease |
| Prompt | A 39-year-old woman presents to her primary care physician complaining of fever, muscle soreness, headache, and severe malaise one week after returning from a summer vacation on Cape Cod. Physical examination is remarkable for a red rash on her left leg with an inner ring clearing resembling a bullseye. What is the most likely causal organism? |
| Answer A | Borrelia burgdorferi |
| Answer A Explanation | [[AnswerAExp::Borrelia burgdorferi causes Lyme disease.]] |
| Answer B | Rickettsia rickettsii |
| Answer B Explanation | Rickettsia rickettsii causes Rocky Mountain Spotted Fever (RMSF). |
| Answer C | Coxiella burnetii |
| Answer C Explanation | [[AnswerCExp::Coxiella burnetii causes Q Fever.]] |
| Answer D | Brucella spp. |
| Answer D Explanation | [[AnswerDExp::Brucella causes a triad of undulant fever, arthritis, and wet-hay smelling sweat)]] |
| Answer E | Francisella tularensis |
| Answer E Explanation | [[AnswerEExp::Francisella tularensis causes tularemia.]] |
| Right Answer | A |
| Explanation | The patient in this vignette has the early (primary) stage of Lyme disease, caused by the spirochete Borrelia burgdorferi. Borrelia burgdorferi is a zoonotic bacteria that is carried by the Ixodes (deer) tick. In its early stages, Lyme disease causes flu-like symptoms and a characteristic “bullseye” rash called erythema chronicum migrans present in 80% of patients. The incubation period of Lyme disease can vary greatly, but is typically 1 to 2 weeks.
If untreated, Lyme disease can cause facial palsy and AV nodal block (secondary Lyme disease) progressing to chronic monoarthritis or migratory polyarthritis (tertiary Lyme disease). It is endemic to Northeastern USA. Treatment is generally by doxycyline. |
| Approved | Yes |
| Keyword | Microbiology, Zoonotic, Animal, Tick, Lyme disease, Lyme, Bacteria, Rash, Dermatology, Linked |
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