A 55-year-old woman comes to the physician with a 6-month history of cough and dyspnea. She has smoked 1 pack of cigarettes daily for the past 30 years. Analysis of the sputum sample from bronchoalveolar lavage shows abnormal amounts of an isoform of elastase that is normally inhibited by alpha-1 antitrypsin. The cell responsible for secreting this elastase is most likely also responsible for which of the following functions?

Phagocytosis of foreign material

A 14-year-old girl is brought to the emergency department because of a 3-day history of worsening confusion, high-grade fever, and a productive cough. She has had recurrent respiratory infections and bulky, foul-smelling, oily stools since infancy. She is at the 14th percentile for height and 8th percentile for weight. Despite appropriate care, the patient dies 2 days after admission. Autopsy of the lungs shows bronchial mucus plugging and bronchiectasis. Which of the following is the most likely underlying cause of this patient's condition?

Deletion of phenylalanine codon on chromosome 7

Mutation of DNAI1 gene on chromosome 9

Deficiency in alpha-1 antitrypsin

Deficiency in apolipoprotein B-48

Deficiency in adenosine deaminase

An 11-year-old boy who recently emigrated from Ukraine is brought to the physician for the evaluation of failure to thrive. Genetic analysis shows the deletion of the 508th codon in a gene on chromosome 7. The deletion results in defective post-translational folding of a protein and retention of the misfolded protein in the rough endoplasmic reticulum. The activity of which of the following channels is most likely to be increased as a result of the defect?

Sodium channels of respiratory epithelial cells

Calcium channels of distal tubular cells

ATP-sensitive potassium channels of pancreatic beta cells

Bicarbonate channels of pancreatic ductal cells

Chloride channels of epithelial cells in sweat glands

A 9-year-old boy with cystic fibrosis (CF) presents to the clinic with fever, increased sputum production, and cough. The vital signs include: temperature 38.0°C (100.4°F), blood pressure 126/74 mm Hg, heart rate 103/min, and respiratory rate 22/min. His physical examination is significant for short stature, thin body frame, decreased breath sounds bilateral, and a 2/6 holosystolic murmur heard best on the upper right sternal border. His pulmonary function tests are at his baseline, and his sputum cultures reveal Pseudomonas aeruginosa. What is the best treatment option for this patient?

Dornase alfa 2.5 mg as a single-use

Sulfamethoxazole and trimethoprim for 14 days

A 7-year-old boy with a history of cystic fibrosis is brought to the physician for evaluation of recurrent episodes of productive cough, wheezing, and shortness of breath over the past month. Physical examination shows coarse crackles and expiratory wheezing over both lung fields. Serum studies show elevated levels of IgE and eosinophilia. A CT scan of the lungs shows centrally dilated bronchi with thickened walls and peripheral airspace consolidation. Antibiotic therapy is initiated. One week later, the patient continues to show deterioration in lung function. A sputum culture is most likely to grow which of the following?

Monomorphic, septate hyphae that branch at acute angles

Monomorphic, broad, nonseptate hyphae that branch at wide angles

Dimorphic, broad-based budding yeast

Dimorphic, cigar-shaped budding yeast

Monomorphic, narrow budding encapsulated yeast

Biofilms in cystic fibrosis for USMLE Step 1: High-Yield Microbiology Lessons

Biofilms & CF - The Slimy Setup

Foundation: Defective CFTR protein → thick, dehydrated airway mucus. This static mucus layer is a perfect breeding ground for bacteria, especially Pseudomonas aeruginosa.
Mechanism: Bacteria colonize the mucus, communicate via quorum sensing, and form a protective extracellular polysaccharide matrix (EPS).

P. aeruginosa biofilm in cystic fibrosis lung tissue

Consequences:
- ↑ Antibiotic resistance (physical barrier).
- Chronic, difficult-to-eradicate infections.
- Persistent inflammation & progressive lung damage.

⭐ High-Yield: The mucoid phenotype of P. aeruginosa in CF is due to alginate overproduction, a key component of the biofilm matrix that shields the bacteria from host defenses and antibiotics.

Key Pathogens - The Usual Suspects

Pseudomonas aeruginosa: The classic CF pathogen, notorious for forming biofilms.
- Transitions from non-mucoid to a mucoid phenotype by overproducing alginate, a key biofilm component.
- This conversion marks the shift to chronic, persistent infection.
Staphylococcus aureus: A common early colonizer, often preceding Pseudomonas.
- Methicillin-resistant S. aureus (MRSA) is a growing concern.
Burkholderia cepacia complex: Less common but associated with severe necrotizing pneumonia ("cepacia syndrome") and poor outcomes.
Other significant pathogens:
- Haemophilus influenzae (non-typeable)
- Aspergillus fumigatus (fungal)

P. aeruginosa biofilm in cystic fibrosis airways

⭐ The mucoid, alginate-producing phenotype of P. aeruginosa is a major virulence factor, conferring significant resistance to both antibiotics and host immune clearance.

Pathogenesis - The Vicious Cycle

Vicious cycle of lung pathophysiology in cystic fibrosis

A cycle of infection, inflammation, and tissue damage perpetuates lung disease in CF.
Initial colonization (often P. aeruginosa) → Biofilm formation → Chronic inflammation.
Host response is dominated by a massive, ineffective influx of neutrophils.
Neutrophils undergo necrosis/NETosis, releasing DNA, proteases (elastase), and oxidants that damage airways and contribute to the biofilm matrix.

⭐ The DNA released from necrotic neutrophils is a key structural component of the biofilm matrix. Dornase alfa (DNase) therapy targets this extracellular DNA, cleaving it to reduce sputum viscosity.

Management - The Uphill Battle

Aggressive Antibiotic Therapy:
- Primary strategy involves high-dose, prolonged, and often cyclical courses targeting P. aeruginosa.
- Inhaled antibiotics (e.g., tobramycin, aztreonam) are crucial for delivering high concentrations directly to the lung with minimal systemic toxicity.
- Combination therapy is standard during exacerbations.
Biofilm & Mucus Disruption:
- Dornase alfa (DNase): An enzyme that cleaves extracellular DNA in the biofilm matrix, reducing sputum viscosity.
- Hypertonic saline: Improves airway surface hydration and facilitates mucociliary clearance.

⭐ Azithromycin is used chronically not for its bactericidal effect, but for its anti-inflammatory properties and its ability to inhibit bacterial quorum sensing, a key process in biofilm maintenance.

High‑Yield Points - ⚡ Biggest Takeaways

Cystic Fibrosis (CF), from CFTR gene mutations, features thick mucus that fosters biofilm growth.

Pseudomonas aeruginosa is the hallmark pathogen, switching to a mucoid phenotype by overproducing alginate.

The biofilm matrix provides a shield, leading to high antibiotic resistance and immune evasion.

Quorum sensing orchestrates gene expression, controlling virulence factors within the biofilm.

Chronic biofilm presence drives persistent inflammation, bronchiectasis, and progressive lung function decline.

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