How does montelukast (Singulair), a leukotriene blocker, work in the treatment of asthma?

Choice A reason: This is not the definition of ejection fraction. The percentage of blood that leaves the heart in a minute is related to the cardiac output, which is the product of the heart rate and the stroke volume.

Choice B reason: This is the correct definition of ejection fraction. It is the ratio of the stroke volume (the amount of blood pumped by the left ventricle) to the end-diastolic volume (the amount of blood in the left ventricle at the end of diastole). It is a measure of the efficiency of the heart and reflects the contractility of the cardiac muscle.

Choice C reason: This is not the definition of ejection fraction. The amount of blood that leaves the heart with each beat is the stroke volume, which is one of the factors that determine the ejection fraction.

Choice D reason: This is not the definition of ejection fraction. The amount of blood that leaves the heart in a minute is the cardiac output, which is influenced by the ejection fraction, but not equivalent to it.

Choice A reason: Hyperplasia and deformation of bronchial cartilage are not the causes of airway obstruction in COPD type B. Bronchial cartilage is the rigid structure that supports the bronchi, the large airways that branch from the trachea. Hyperplasia is an increase in the number of cells, and deformation is a change in the shape or structure of the cells. These processes can affect the bronchial cartilage, but they do not directly obstruct the airway.

Choice B reason: Loss of alveolar elastin is not the cause of airway obstruction in COPD type B. Alveolar elastin is the elastic fiber that allows the alveoli, the tiny air sacs at the end of the bronchioles, to expand and recoil during breathing. Loss of alveolar elastin is a characteristic of COPD type A (emphysema), which causes the alveoli to lose their shape and collapse. This reduces the surface area for gas exchange, but it does not obstruct the airway.

Choice C reason: Pulmonary edema is not the cause of airway obstruction in COPD type B. Pulmonary edema is the accumulation of fluid in the lungs, usually due to heart failure or lung injury. It causes shortness of breath, coughing, and crackles in the lungs. It can impair gas exchange and oxygenation, but it does not obstruct the airway.

Choice D reason: Thick mucus, fibrosis, and smooth muscle hypertrophy are the causes of airway obstruction in COPD type B. Thick mucus is the result of chronic inflammation and infection of the bronchi, which stimulates the mucus glands to produce more and thicker mucus. Fibrosis is the formation of scar tissue in the bronchial walls, which narrows the airway and reduces its elasticity. Smooth muscle hypertrophy is the enlargement of the smooth muscle cells that surround the bronchi, which increases the airway resistance and causes bronchospasm. These processes combine to obstruct the airway and cause chronic cough, wheezing, and dyspnea.