A 70-year-old woman is brought to the emergency department by her daughter due to 2 days of fever and cough and a day of excessive somnolence. The patient has a history of hypertension and osteoarthritis. She does not use tobacco or alcohol. She lives independently in a senior residential facility and ambulates with the assistance of a cane. Temperature is 39.2 C (102.6 F), blood pressure is 110/76 mm Hg, pulse is 110/min, and respirations are 24/min. No pharyngeal erythema or cervical lymphadenopathy is present. Lung auscultation reveals crackles in the right lower lung field. Laboratory results are as follows: Chest x-ray reveals a right lower lobe infiltrate. Which of the following pathogens is the most likely cause of this patient's condition?

Question

A 70-year-old woman is brought to the emergency department by her daughter due to 2 days of fever and cough and a day of excessive somnolence.  The patient has a history of hypertension and osteoarthritis.  She does not use tobacco or alcohol.  She lives independently in a senior residential facility and ambulates with the assistance of a cane.  Temperature is 39.2 C (102.6 F), blood pressure is 110/76 mm Hg, pulse is 110/min, and respirations are 24/min.  No pharyngeal erythema or cervical lymphadenopathy is present.  Lung auscultation reveals crackles in the right lower lung field.  Laboratory results are as follows:  Chest x-ray reveals a right lower lobe infiltrate.  Which of the following pathogens is the most likely cause of this patient's condition?

Quizplus · Accepted Answer

Explanation: This elderly patient with fever, cough, tachypnea, leukocytosis, and chest x-ray evidence of a lobar infiltrate likely has pneumonia. According to the most recent guidelines published by the Infectious Diseases Society of America, pneumonia is classified as follows: Community-acquired - develops in a nonhospitalized setting Hospital-acquired - develops ≥48 hours after admission to the hospital Ventilator-acquired - develops ≥48 hours after endotracheal intubation Health care-associated pneumonia is no longer an active designation for pneumonia as it poorly predicts those at risk for multi-drug resistant organisms. As this patient developed pneumonia while residing at an assisted-living facility (a nonhospital setting), she has community-acquired pneumonia (CAP). The most common cause of CAP is Streptococcus pneumoniae. This organism is acquired by aerosol inhalation and colonizes the nasopharynx. Most patients remain asymptomatic and clear the inoculation within 4-6 weeks. However, those with aggressive serotypes or certain underlying risk factors (eg, age >65, immunosuppression) are more likely to develop invasive disease (eg, pneumonia, meningitis, endocarditis). (Choice A) Anaerobic lung infections most commonly occur in individuals with alcoholism or dysphagia. Although the dependent portion of the lung (eg, right lower lobe) is often affected, most patients have foul-smelling sputum and symptoms that progress over 1-2 weeks. This patient's rapid-onset illness and lack of aspiration risk factors makes anaerobic infection less likely. (Choice B) Haemophilus influenzae is a common cause of upper airway infections (eg, sinusitis, bronchitis, CAP). H influenzae can cause CAP but is less common than S pneumoniae. (Choice C) Pseudomonas aeruginosa is a common cause of hospital-acquired pneumonia but a rare cause of CAP. Patients with immunocompromise or structural lung disease (eg, cystic fibrosis) can sometimes develop P aeruginosa in the community setting. (Choice E) Streptococcus pyogenes primarily causes skin infections (eg, erysipelas, cellulitis) and pharyngitis. It is not a typical cause of CAP. Educational objective: Streptococcus pneumoniae is the most common cause of community-acquired pneumonia. Manifestations tend to begin abruptly and include fever, chills, productive cough, leukocytosis, and chest x-ray evidence of a lobar infiltrate. References: Community-acquired pneumonia in the elderly. (http://www.ncbi.nlm.nih.gov/pubmed/20226392) Impact of guideline-consistent therapy on outcome of patients with healthcare-associated and community-acquired pneumonia. (http://www.ncbi.nlm.nih.gov/pubmed/21586592)

A 70-Year-Old Woman Is Brought to the Emergency Department by Her