The answer is A. Pertussis
Weeks of paroxysmal coughing with an inspiratory whoop and post-tussive vomiting, a well-appearing afebrile child between spells, and a marked lymphocyte-predominant leukocytosis is pertussis. Bordetella pertussis is a gram-negative coccobacillus that attaches to respiratory ciliated epithelium and releases pertussis toxin, which paralyzes the mucociliary escalator and drives the lymphocytosis. Understanding that mechanism explains the whole clinical course: the airway cannot clear its own secretions, so the child coughs in long uninterrupted runs to move mucus, and the whoop is the sound of air rushing back through a narrowed glottis after the lungs have been emptied. Learn the disease in its three stages, because the stage determines both the differential and whether antibiotics help. The catarrhal stage lasts one to two weeks and looks exactly like a cold — rhinorrhea, mild cough, little or no fever — and this is when the child is most contagious and when treatment is most effective, which is the central tragedy of pertussis, because nobody suspects it yet. The paroxysmal stage runs two to six weeks and delivers the classic picture: fits of coughing, whoop, post-tussive emesis, and the mechanical consequences of violent coughing such as subconjunctival hemorrhage, petechiae above the nipple line, and rarely rib fracture or pneumothorax. The convalescent stage is weeks to months of a gradually improving cough, which is why the illness is nicknamed the hundred-day cough. Two exam-favorite discrepancies: the examination and chest film are normal despite dramatic symptoms, and fever is absent or minimal. Diagnosis is by nasopharyngeal PCR, which is the test of choice; culture on Bordet-Gengou or Regan-Lowe medium is highly specific but slow and insensitive after the first two weeks, and serology is useful late in the course. Treatment is a macrolide — azithromycin preferred, with erythromycin or clarithromycin as alternatives, and trimethoprim-sulfamethoxazole for macrolide intolerance in children over two months. Understand what antibiotics actually accomplish: given in the catarrhal stage they can shorten the illness, but once paroxysms are established they mainly eliminate carriage and stop transmission rather than change the cough — treat anyway, for public health. Give the same macrolide as post-exposure prophylaxis to all household and close contacts regardless of their immunization status, and report the case to public health. Prevention is the DTaP series in childhood with a Tdap booster at 11 to 12 years, and Tdap in every pregnancy between 27 and 36 weeks to give the newborn transplacental antibody. Infants under 6 months are the group that dies — they may present with apnea, cyanosis, or bradycardia and no whoop at all — and they need hospitalization. Sorting the distractors: Mycoplasma pneumoniae causes a persistent dry cough in a school-aged child but with malaise, low-grade fever, and patchy interstitial infiltrates on a film that looks worse than the patient, and it produces no whoop or lymphocytosis; cough-variant asthma is chronic cough triggered by exercise, cold air, or allergens, often nocturnal, responsive to bronchodilators, without a preceding catarrhal illness, vomiting, or leukocytosis; foreign body aspiration is a sudden choking event with focal unilateral wheeze or decreased breath sounds and possible hyperinflation on expiratory films, not a four-week staged illness; and respiratory syncytial virus bronchiolitis is a disease of infants under two, with fever, tachypnea, diffuse wheezes and crackles, and hyperinflation — not a well-appearing 9-year-old with clear lungs.
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