The commonest cause of lung abscess-

Ans. is 'd' aspirated oropharyngeal secretion Harrison writes - 'Risk factors for lung abscess include conditions associated with impaired cough reflex and/or aspiration, such as alcoholism, anaesthesia, drug abuse, epilepsy, and stroke. Dental caries, bronchiectasis, *bronchial carcinoma and pulmonary infarction are also risk factors.Most aspiration - associated lung abscesses are due to a combination of aerobic and anaerobic bacteria, with an average of six or seven bacterial species identified in an individual case.The implicated anaerobic bacteria include the Bacteroides fragilis group. Bacteroides gracilis, Prevotella intermedia, Prevotella denticola, Prevotella melaninogenicus, Prevotella oralis, Fusobacterium nucleatum, Peptostreptococcus micros, Peptostreptococcus anaerobius, and Peptostreptococcus magnus.Streptococcus milleri is one of the principal aerobic pathogens; S. aureus, S. pneumoniae, H. influenzae, P. aeruginosa, E. coli, and Klebsiella pneumoniae are also isolated frequently. Rarely S. pneumoniae alone (usually capsular type 111) can cause lung abscess.In HIV-infected patients, lung abscess can be due to Pneumocystis, Rhodococcus equi. and cryptococcus neoformans as well as the bacteria noted above. "