STEP 1 Question of the Month (March)

March 31, 2025 |

Conquer STEP 1, One Question at a Time! The answer is explained below.

A 4-year-old male is brought to a clinic because of a 1-week history of fever, cough, and runny nose and a 2-day history of intense coughing spells that last 1 to 2 minutes, occasionally followed by vomiting and dry heaving. His mother says he has not received any immunizations. Temperature is 38.5°C (101.3°F). WBC count is 22,000/mm³ with 60% lymphocytes.

Which of the following pathogens is the most likely cause of this patient’s symptoms?

A) Bordetella pertussis

B) Corynebacterium diphtheriae

C) Haemophilus influenzae

D) Measles virus

E) Streptococcus pneumoniae

Check the answer below 👇

The correct answer is: A) Bordetella pertussis.

High-Yield Summary

Pertussis, or whooping cough, presents as initial catarrhal coldlike symptoms, followed by a paroxysmal phase with violent, distinctive coughing (whoops) that can last several weeks.
Whooping cough is caused by the gram-negative bacterium B pertussis.

Step 1: Disease Diagnosis

Initial coldlike symptoms followed by intensive coughing spells are consistent with whooping cough, or pertussis. There is leukocytosis with lymphocytic dominance, as seen here. Initially resembling an upper respiratory infection (catarrhal phase), the infection progresses into the paroxysmal phase, which is characterized by distinctive forced intake of breath, or whoops. This child is currently in the paroxysmal phase.

Disease Diagnosis: Bordetella pertussis

Whooping cough is caused by the gram-negative bacterium Bordetella pertussis, and infection is vaccine-preventable. The DTaP vaccine contains acellular pertussis antigens; the “aP” component of the vaccine is several B pertussis toxins and filamentous hemagglutinin.

The other choices are incorrect:

Measles virus presents with a cough, coryza, conjunctivitis, high fever, and Koplik spots on the buccal mucosa, followed by a characteristic rash with a downward spread.
Streptococcus pneumoniae presents with a productive cough with blood-tinged sputum. Infection is generally localized to the lower lobes of the lungs.
Corynebacterium diphtheriae manifests as a pharyngitis that progresses to a pharyngeal pseudomembrane.
Nontypable strains of Haemophilus influenzae may cause upper respiratory tract infections such as acute and chronic otitis and sinusitis. Typeable strains, such as H influenzae type B, can cause epiglottitis, pneumonia, and meningitis in unimmunized individuals.

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