Review Question - QID 100939

This patient died from lobar pneumonia. The red hepatization phase is grossly characterized by a red, firm lobe with a "liver-like" consistency, that corresponds microscopically to a massive confluent alveolar exudate containing neutrophils, RBCs, and fibrin.

Pneumonia (PNA) can be classified as 1 of 3 morphological types: bronchopneumonia (dispersed patchy inflammation starting in the bronchioles and involving the alveoli), interstitial PNA (inflammation of the alveolar walls), or lobar PNA (inflammatory process involving an entire lobe of the lung). Lobar PNA has 4 characteristic stages: congestion, red hepatization, grey hepatization, and resolution.

Watkins et al. discuss the diagnosis and treatment of community acquired pneumonia (CAP). The definition of CAP is an infection of the lung not acquired in a hospital, long-term care facility, or through recent contact with the health care system. Although many pathogens can cause CAP, the most common known eitiology for outpatients is Mycoplasma pneumoniae, while Streptococcus pneumoniae is the most common in patients admitted to the hospital. Since a causative organism is commonly not identified in patients with CAP, treatment is generally empiric against most likely organisms.

Blake et al. studied the pathology and pathogenesis of pneumococcal lobar PNA in monkeys by injecting them intratracheally with pneumococcus. They found that pneumococcus primarily invades the pulmonary tissue proximal to the hilum and spreads rapidly throughout the lobe, interstitial tissue, and alveolar structures. Hepatization begins centrally and spreads toward the periphery in a constantly progressive process. Resolution is frequently accompanied by a varying degree of organization, leaving behind fibrous thickening or permanent adhesions.

Illustration A depicts the 4 stages of lobar pneumonia.
Illustration B depicts a general overview of lobar pneumonia.
Illustration C shows a typical radiograph demonstrating lobar pneumonia (likely left lower lobe in this example).

Incorrect Answers:
Answer 1: Acute eosinophilic PNA is characterized by respiratory distress, eosinophilic infiltration in the lung, acute onset, resolution of symptoms with corticosteroids and the absence of relapse.
Answer 2: Vascular dilation is not a common manifestation of PNA. Noncaseating granulomas are often seen in sarcoidosis, not PNA.
Answer 3: Invasive Streptococcus PNA is an uncommon cause of hemolytic uremic syndrome which can cause fragmented erythrocytes. This would not be expected in a typical case of PNA. Fragmented red blood cells are also seen in grey hepatization.
Answer 5: Whorls of collagen in histological examination are highly indicative of silicosis.