The third stage of gray hepatization stage occurs 2-3 days later, and the lung appears dark brown. Fibrin deposits in the alveoli are common. The stage is characterized by neutrophils, red blood cells, and desquamated epithelial cells. Red hepatization is the second stage, and it has the consistency of the liver. Both bacteria and neutrophils are present. There are four stages of lobar pneumonia. The first stage occurs within 24 hours and is characterized by alveolar edema and vascular congestion. In the majority of cases, the microbe is not identified, and the most common cause is of viral etiology. This is ultimately accompanied by a migration of inflammatory cells to the site of infection, causing an exudative process, which in turn impairs oxygenation. Once these barriers are breached, infection, either by fomite/droplet spread (mostly viruses) or nasopharyngeal colonization (mostly bacterial), results in inflammation and injury or death of surrounding epithelium and alveoli. There are barriers to infection that include anatomical structures (nasal hairs, turbinates, epiglottis, cilia), and humoral and cellular immunity. Pneumonia is an invasion of the lower respiratory tract, below the larynx by pathogens either by inhalation, aspiration, respiratory epithelium invasion, or hematogenous spread. Unvaccinated children are at risk for vaccine-preventable pathogens. Children who are immunocompromised should be evaluated for Pneumocystis jirovecii, cytomegalovirus, and fungal species if no other organism is identified. Patients with sickle cell disease are at risk of infection from encapsulated organisms. ![]() aureus and Pseudomonas aeruginosa is ubiquitous. In cystic fibrosis, pneumonia secondary to S. Children with chronic diseases are also at risk for specific pathogens. It is important to consider tuberculosis (TB) in immigrants from high prevalence areas, and children with known exposures. Adolescents usually have the same infectious risks as adults. pneumoniae is still the most commonly identified organism. Mycoplasma pneumonia frequently occurs in children in the range from 5 to 13 years old however, S. influenzae type B is observed in this age group. ![]() In children 2 to 5 years old, respiratory viruses are also the most common. Viruses are the main cause of pneumonia in older infants and toddlers between 30 days and 2 years old. Streptococcus pneumoniae, Streptococcus pyogenes, and Staphylococcus aureus can be identified in late-onset neonatal pneumonia. Neonates are at risk for bacterial pathogens present in the birth canal, and this includes organisms such as group B streptococci, Klebsiella, Escherichia coli, and Listeria monocytogenes. The etiology of pneumonia in the pediatric population can be classified by age-specific versus pathogen-specific organisms.
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