Fetal response to Treponema pallidum occurs at which week of pregnancy?
**Core Concept**
Fetal response to Treponema pallidum, the causative agent of syphilis, occurs due to the transplacental passage of the bacteria from the mother to the fetus. This results in congenital syphilis, a condition characterized by various systemic manifestations.
**Why the Correct Answer is Right**
The fetal response to Treponema pallidum typically occurs after the 16th week of gestation, as the placenta develops and allows for the transplacental passage of the bacteria. This passage can lead to the infection of the fetus, resulting in congenital syphilis. The fetal response is thought to be mediated by the immune system's reaction to the invading bacteria.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify a gestational age, making it an ambiguous answer.
**Option B:** This option is incorrect as it suggests the fetal response occurs too early in pregnancy, before the placenta has fully developed.
**Option C:** This option is incorrect as it suggests the fetal response occurs too late in pregnancy, after the 28th week.
**Clinical Pearl / High-Yield Fact**
It is essential to note that the risk of congenital syphilis is highest in pregnancies where the mother's syphilis is not diagnosed or treated, emphasizing the importance of regular prenatal care and syphilis screening.
**Correct Answer: C. 28th week.**