October 25, 2010

USMLE 1 Question 16

A 60yr old farmer presents with a chronic cough and progressive dyspnea for the last 6yrs. The chest x-ray revealed a reticulonodular infiltrate associated with honey-combing of the lung.

Pulmonary function studies show a restrictive pattern with loss of lung volumes, impaired diffusion capacity, decreased compliance, and an exercise-induced hypoxemia. Serum studies demonstrate precipitating antibodies against extracts of moldy hay.

The causative organism in this case is likely to be:

a) Aspergillus
b) Thermophillic actinomycetes
c) Pencillium casei
d) Bacillus subtillis
e) Bacillus anthracis

The correct answer is B

The pulmonary function tests are suggestive of a restrictive lung disease. The chest X-ray consisitng of reticulonodular infiltrates and honey combing of the lung also point towards a restrictive lung disease most likely interstitial pneumonitis.

The presence of precipitating antibodies against extracts of moldy hay and the man's occupation clinches the diagnosis as that of a Farmer's Lung.

Farmer's lung is a rare kind of occupational disease seen in farmers due to exposure to dusts generated from harvested, humid warm hay that permits the rapid proliferation of spores of thermophillic actinomycetes.

It is a type of hypersensitivity pneumonitis in which interstitial lung involvement occurs. The disease has an insidious course over years associated with chronic cough and progressive dyspnea. End stage lung disease is characterized by a fibrotic lung with reticulonodular infiltrates and honey combing of the lung.

Aspergillus is responsible for a variety of occupational and other interstitial lung diseases with prolonged symptoms but it is not the causative agent of farmer's lung.

Pencillium casei is responsible for cheese washer's lung. Bacillus subtillis is responsible for detergent worker's disease. Bacillus anthracis is responsible for pulmonary anthrax which is a very acute disease resulting in death within days if not treated. It also causes parenchymal pneumonia rather than interstitial lung involvement.

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