What are common causes of septic arthritis in infants up to 3 months old?

In infants up to 3 months old, the common causes of septic arthritis primarily include Streptococcus agalactiae (Group B Streptococcus), Staphylococcus aureus, and Gram-negative bacilli. This demographic is particularly vulnerable to these pathogens due to a combination of factors, including their immature immune systems and the fact that they can acquire infections from maternal sources (particularly Group B Streptococcus during delivery).

Group B Streptococcus is a leading cause of neonate sepsis and can lead to septic arthritis, especially in those with risk factors such as prematurity or prolonged rupture of membranes. Staphylococcus aureus is a versatile pathogen that can cause a range of infections, including those in very young infants. Additionally, Gram-negative bacilli, which can be part of the normal flora in the gastrointestinal tract and can lead to infections if they enter the bloodstream or joints, are also significant contributors to septic arthritis in this age group.

The other options include organisms that either do not commonly cause septic arthritis in infants or are more relevant in older children and adults. Therefore, option B accurately reflects the typical pathogens observed in cases of septic arthritis in this vulnerable age group.