Respiratory pathogens most commonly reach the lower respiratory tract by:
a. Person-to-person transmission
b. Micro-aspiration from the upper respiratory tract
c. Hematogenous spread
d. Failure of the mucociliary escalator
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Answer: B. The development of pneumonia requires that a pathogen reach the alveoli and that host defenses are overwhelmed by either the micro-organism’s virulence or by the inoculum’s size. The pathogens that cause pneumonia can be transmitted from person to person by airborne droplets, but they may already be present in native oral and nasal flora. The most common way that these pathogens reach the lower respiratory tract is by micro-aspiration. Conditions such as chronic lung disease, smoking, older age, and immunocompromise can predispose patients to the development of pneumonia. Younger or healthier patients can typically mount an effective immune response when small numbers of low-virulence microbes are deposited in the lungs.