WBR0048

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Author [[PageAuthor::William J Gibson (Reviewed by Rim Halaby, M.D. [1] and Yazan Daaboul)]]
Exam Type USMLE Step 1
Main Category Microbiology
Sub Category Pulmonology, General Principles
Prompt A 5-year-old boy is brought to his pediatrician by his mother for a 12-day history of worsening dry cough. His blood pressure is 110/78 mmHg, heart rate is 108/min, respiratory rate is 98%, and temperature is 38.7 °C (101.6 °F). During physical examination, the patient experiences paroxysmal coughing spells and is temporarily unable to breathe. Further history reveals the child immigrated from rural India with his family 3 months ago and has not received any medical care before today's visit. Initial lab tests are significant for marked lymphocytosis. Which of the following media could be used to culture the infectious agent of this patient's condition?
Answer A Eaton’s Agar
Answer A Explanation [[AnswerAExp::Eaton’s agar is used to culture Mycoplasma pneumoniae.]]
Answer B Charcoal yeast extract
Answer B Explanation [[AnswerBExp::Charcoal yeast extract is used to culture Legionella pneumophila.]]
Answer C Tellurite agar
Answer C Explanation [[AnswerCExp::Tellurite agar is used to culture Corynebacterium diphtheriae.]]
Answer D Bordet-Gengou agar
Answer D Explanation [[AnswerDExp::Bordetella pertussis may be cultured on Bordet-Gengou Agar.]]
Answer E Lowenstein-Jensen
Answer E Explanation [[AnswerEExp::Lowenstein-Jensen agar is used to culture Mycobacterium tuberculosis.]]
Right Answer D
Explanation [[Explanation::Pertussis, or whooping cough, is an infection caused by Bordetella pertussis. The patient’s immigration history suggests that he did not receive routine vaccinations, placing him at higher risk for whooping cough, a very rare infection in USA and other developed countries due to the practice of routine vaccination (part of DTaP vaccine). Whooping cough is characterized by fits of paroxysmal coughing, and in 50% of patients an inspiratory “whoop” that can be appreciated as the patient attempts to draw air through a partially closed glottis. B. pertussis is a gram-negative, aerobic coccobacillus, which can be cultured on Bordet-Gengou agar.

It is a highly infectious organism that is transmitted by direct contact of respiratory secretions or via airborne transmission of infected secretions.

Pertussis has 3 stages:

  • Catarrhal stage (7-10 days): Non-specific symptoms, such as mild cough, rhinorrhea, and low-grade fever. Patients are most contagious in this stage.
  • Paroxysmal stage (2-3 weeks): Paroxysmal coughing spells, especially at night
  • Convalescent stage (several weeks): Gradual improvement of symptoms

Lymphocytosis is frequently observed among patients with pertussis because pertussis toxin (PT), or lymphocyte-promoting factor (LPF), induced by the organism, causes lymphocytosis, IL-4, and IgE secretion due to impaired entry of lymphocytes into lymph nodes and blockage of lymphocyte extravasation. The treatment of pertusses includes supportive therapy and antibiotics, especially early in the course of the disease. Antibiotics may not be helpful if prescribed beyond 3 weeks of illness. Other household family members may be prescribed a 14-day course of prophylactic erythromycin.
Educational Objective: Bordetella pertussis causes whooping cough and is cultured on Bordet-Gengou agar.
References: Mu HH, Cooley MA, Sewell WA. Studies on the lymphocytosis induced by pertussis toxin. Immunol Cell Biol. 1994;72(3):267-70. First Aid 2014 page 122]]

Approved Yes
Keyword Microbiology, Respiratory, Cough, Bacteria, Laboratory
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