Final answer:
Pneumonia is the disease agent from the given options that is unable to cross the placenta and infect the fetus, as syphilis, rubella, and AIDS have mechanisms that allow for placental transmission.
Step-by-step explanation:
The disease agent unable to cross the placenta to infect the fetus from the choices provided is pneumonia. Diseases like syphilis, rubella, and AIDS caused by the human immunodeficiency virus (HIV) have mechanisms that can enable them to cross the placental barrier and potentially infect the fetus. Pneumonia, which can be caused by various pathogens such as bacteria, viruses, and fungi, usually does not have this capacity. The placental barrier typically blocks the transmission of pneumonia-causing agents from the mother to the fetus, thus not allowing the disease to affect the unborn child directly through vertical transmission.
It is important to note that the designation pneumonia encompasses a range of infections, and while bacterial agents such as Streptococcus pneumoniae are major causes, they usually do not infect the fetus. However, the prenatal health of the mother can indirectly impact the fetus, so maternal health is always of utmost importance during pregnancy.