**Question:** A 23-year-old man is experiencing a flare of his asthma. He is using his salbutamol inhaler more frequently than usual and despite increasing his inhaled steroids he is still short of breath. Previously his asthma was considered mild with no severe exacerbations requiring oral steroids or hospitalization. With his flare, he has recurrent episodes of bronchial obstruction, fever, malaise, and expectoration of brownish mucous plugs. On examination, there is bilateral wheezing. The heart, abdomen, neurologic, and skin exams are normal. CXR reveals upper lobe pulmonary infiltrates; the eosinophil count is 3000/mL, and serum precipitating antibodies to Aspergillus are positive. Which of the following is the most likely diagnosis?

A. Allergic bronchopulmonary aspergillosis
B. Allergic bronchitis
C. Pneumonia
D. Bronchiectasis

**Correct Answer:**

A. Allergic bronchopulmonary aspergillosis (ABPA)

Core Concept: Allergic bronchopulmonary aspergillosis (ABPA) is a severe asthma exacerbation caused by hypersensitivity reactions to Aspergillus species, which are commonly found in the respiratory tract. ABPA occurs in individuals with a background of asthma and a history of allergy or atopy.

Explanation:

The patient's clinical presentation aligns with ABPA due to the following factors:

1. Recurrent episodes of bronchial obstruction, fever, and expectoration of brownish mucus plugs are indicative of an allergic reaction and infection.
2. The bilateral wheezing on examination suggests lung involvement.
3. The CXR showing upper lobe pulmonary infiltrates indicates focal lung inflammation.
4. The elevated eosinophil count (3000/mL) is a classic feature of ABPA, as eosinophilia is a characteristic feature of the disease.
5. Positive serum precipitating antibodies to Aspergillus are a key laboratory finding in ABPA, supporting the diagnosis.

Why the Correct Answer is ABPA:

A. Allergic bronchopulmonary aspergillosis is the most likely diagnosis due to the combination of clinical features and laboratory findings mentioned above. This disease is characterized by a hypersensitivity reaction to Aspergillus species, which alters the patient's asthma control, leading to exacerbations and complications like those described.

Why Other Options are Incorrect:

B. Allergic bronchitis (AB) is incorrect because it is a less severe form of asthma exacerbation and does not involve the additional clinical features and laboratory findings mentioned above.

C. Pneumonia is incorrect because pneumonia is a bacterial infection, while the patient's symptoms and findings indicate an allergic reaction to Aspergillus species, not a bacterial infection.

D. Bronchiectasis refers to irreversible bronchial dilation and is not related to the patient's history of asthma exacerbation, fever, brownish sputum, and positive Aspergillus antibodies.