A 33-year-old man suddenly develops severe dyspnea with wheezing. On physical examination, his vital signs are temperature, 37deg C; pulse, 95/min; respirations, 35/min; and blood pressure, 130/80 mm Hg. A chest radiograph shows increased lucency in all lung fields. Arterial blood gas analysis shows PO2, 65 mm Hg; PCO2, 30 mm Hg; and pH, 7.48. A sputum cytologic specimen shows Curschmann spirals, Charcot-Leyden crystals, branching septate hyphae, and eosinophils in a background of abundant mucus. What is the most likely risk factor predisposing him to this illness?

**Core Concept**
The patient's presentation suggests a severe asthma exacerbation, characterized by airway obstruction, hyperinflation of the lungs, and eosinophilic inflammation. The presence of Curschmann spirals and Charcot-Leyden crystals in the sputum further supports this diagnosis.

**Why the Correct Answer is Right**
The patient's history and physical examination findings, combined with the laboratory results, indicate a severe asthma exacerbation. The increased lung lucency on chest radiograph and elevated respiratory rate are consistent with air trapping and hyperinflation. The arterial blood gas analysis shows a mild respiratory alkalosis, likely due to hyperventilation. The presence of eosinophils in the sputum suggests an allergic or atopic component to the asthma. The branching septate hyphae are likely a result of fungal contamination during the sputum processing.

**Why Each Wrong Option is Incorrect**
**Option A:** Allergen exposure is a common risk factor for asthma exacerbations, but it is not the most likely risk factor in this case.
**Option B:** Smoking is a risk factor for chronic obstructive pulmonary disease (COPD), but it is not directly related to asthma exacerbations.
**Option C:** Gastroesophageal reflux disease (GERD) can trigger asthma symptoms in some patients, but it is not the most likely risk factor in this case.
**Option D:** A history of atopy, including allergies, eczema, or hay fever, is a significant risk factor for developing asthma and experiencing exacerbations.

**Clinical Pearl / High-Yield Fact**
Asthma exacerbations can be triggered by a variety of factors, including allergens, respiratory infections, and stress. Identifying and avoiding these triggers can help prevent future exacerbations.

**Correct Answer:** D. A history of atopy is the most likely risk factor predisposing this patient to asthma exacerbations.

✓ Correct Answer: A. Cytokine gene polymorphisms