The shift to humoral immunity during pregnancy increases susceptibility to which of the following infections?

The correct answer is Listeria, which is particularly important to recognize in the context of pregnancy. During pregnancy, there is a natural shift in the immune system that promotes humoral immunity. This shift favors the production of antibodies but can also result in decreased cellular immunity, which is critical for controlling certain intracellular pathogens.

Listeria monocytogenes, a bacterium associated with foodborne infections, can evade the maternal immune response due to this relative decline in cellular immunity. Pregnant women are at a higher risk of listeriosis, which can lead to severe consequences for both the mother and the fetus, including miscarriage, preterm labor, and fetal infection.

The other infections mentioned do pose risks during pregnancy but are typically controlled by the immune response that remains relatively effective. Streptococcus infections, for instance, are commonly encountered and usually can be managed with appropriate medical care. Syphilis and Chlamydia infections can be treated with antibiotics and do not demonstrate the same increased susceptibility as Listeria during pregnancy. Thus, understanding the immune changes during pregnancy helps illuminate why Listeria presents a particular risk during this time, leading to its identification as the correct answer.