Each week, Osmosis shares a USMLE® Step 1-style practice question to test your knowledge of medical topics. Today’s case involves a 30-year-old woman with HIV who has recently developed skin nodules. The woman also works in a pet store. Can you figure it out?
A 30-year-old woman with HIV comes to the clinic due to the recent appearance of skin nodules and progressive fatigue and anorexia for the past 2 months. She recently moved to Ohio and started working in a pet store. She is currently not taking antiretroviral therapy, and her recent CD4 count one month ago was 80 cells/mm3. Her temperature is 38.0°C (100.4°F), her pulse is 80/min, and blood pressure is 125/85 mmHg. Physical examination shows multiple small red-purple papules, some of which are pedunculated and some bleed. Histopathology of one of the lesions shows vascular proliferation with some clusters of neutrophils and lymphocytes. Which of the following is the most likely pathogen responsible for this patient’s presentation?
A. Blastomycosis dermatitis
B. Human herpesvirus 8
C. Bartonella henselae
D. Yersinia pestis
E. Rickettsia rickettsii
Scroll down to find the answer!
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The correct answer to today’s USMLE® Step 1 Question is…
C. Bartonella henselae
Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
The incorrect answers to today’s USMLE® Step 1 Question are…
A. Blastomycosis dermatitis
Incorrect: Blastomycosis dermatitis is a systemic pyogranulomatous infection caused by inhalation of the spores of the dimorphic fungus, which is prevalent in North America. Pulmonary involvement is the most common manifestation, and patients may present with acute or chronic pneumonia. Skin involvement is also common, and presents as gray to violet, verrucous skin lesions with an irregular border that may mimic squamous cell carcinoma. Biopsy from the margins typically shows broad-based budding organisms on microscopy. This patient’s characteristic nodules, biopsy results, and the absence of pulmonary involvement favor another diagnosis.
B. Human herpesvirus 8
Incorrect: Kaposi sarcoma (KS) is an angioproliferative disorder caused by infection with human herpesvirus 8 (HSV-8), often seen in immunocompromised patients. KS can involve virtually any site in the body, however, cutaneous disease is most common. The lesions of KS can be pink, red, purple, or brown, and they tend to be elliptical, not painful or pruritic, and without necrosis. There are generally no associated constitutional symptoms. Additionally, histopathology analysis in KS will likely show whorls of spindle-shaped cells with lymphocytic infiltration, not neutrophilic infiltration, as well as proliferation of small vessels.
D. Yersinia pestis
Incorrect: Yersinia pestis is a zoonotic infection that causes plague. It is transmitted by bites from rodent fleas, scratches or bites from infected domestic cats, and inhalation of respiratory secretions from infected animals. The most common manifestation is acute febrile lymphadenitis (bubonic plague). It is characterized by the acute onset of fever, chills, weakness, and headache, followed by intense pain and swollen lymphadenopathy. Skin lesions may appear and include eschars, pustules, or lesions resembling ecthyma gangrenosum. This patient’s presentation favors another diagnosis.
E. Rickettsia rickettsii
Incorrect: Rickettsia rickettsii is a gram-negative, obligate intracellular bacterium that causes Rocky Mountain spotted fever, a tick-borne disease (Dermacentor dog tick). Patients often present with non-specific symptoms such as fever, headache, malaise, myalgias, and arthralgias followed by blanching erythematous rash with macules that later become petechial. This patient’s presentation favors another diagnosis.
Main Explanation
This patient, who is presenting with fever, fatigue, anorexia, and characteristic skin lesions, most likely has bacillary angiomatosis.
Bacillary angiomatosis is caused by Bartonella species, a fastidious, slow-growing, gram-negative bacillus. The natural reservoir is domestic cats and is transmitted to humans via scratch, bite, or exposure to cat feces/fleas. Cat-scratch disease (CSD) usually affects immunocompetent children and presents as regional limited lymphadenopathy/lymphadenitis. Immunodeficient individuals, however, are at risk for developing disseminated bacillary angiomatosis which usually occurs when CD4 count is less than 100 cells/mm3.
Patients with bacillary angiomatosis often present with constitutional symptoms, including fever, chills, malaise, headache, anorexia, weight loss and unique vascular lesions. The skin is the most affected organ, but the lesions can occur throughout the body (e.g., bone, lymph nodes, GI). The lesions may start as red to purple papules which gradually expand into pedunculated lesions or nodules that become friable and bleed. Biopsy shows vascular proliferation with clusters of neutrophils and lymphocytes infiltrating. Diagnosis is challenging and usually requires a combination of tests including culture, PCR, biopsy, and staining with Warthin-Starry silver staining. Treatment consists of prolonged courses of either erythromycin or doxycycline.
Major Takeaway
Bacillary angiomatosis is caused by Bartonella species, a gram-negative bacilli transmitted to humans, most commonly via scratch from an infected cat. Immunodeficient individuals may present with fever, fatigue, anorexia, and characteristic pedunculated, vascular skin lesions.
References
Saccente M, Woods GL. Clinical and laboratory update on blastomycosis. Clin Microbiol Rev. 2010;23(2):367‐381. doi:10.1128/CMR.00056-09
Lodi S, Guiguet M, Costagliola D, et al. Kaposi sarcoma incidence and survival among HIV-infected homosexual men after HIV seroconversion. J Natl Cancer Inst. 2010;102(11):784‐792. doi:10.1093/jnci/djq134
Prentice MB, Rahalison L. Plague. Lancet. 2007;369(9568):1196‐1207. doi:10.1016/S0140-6736(07)60566-2
LeBoit PE, Berger TG, Egbert BM, Beckstead JH, Yen TS, Stoler MH. Bacillary angiomatosis. The histopathology and differential diagnosis of a pseudoneoplastic infection in patients with human immunodeficiency virus disease. Am J Surg Pathol. 1989;13(11):909‐920.
Cotell SL, Noskin GA. Bacillary angiomatosis. Clinical and histologic features, diagnosis, and treatment. Arch Intern Med. 1994;154(5):524‐528. doi:10.1001/archinte.154.5.524
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