**Core Concept**
Vertical transmission refers to the passage of a pathogen or disease from mother to fetus during pregnancy, which can occur through the placenta or during delivery. The third trimester is a critical period for vertical transmission, as the fetus is more susceptible to infection due to its immature immune system.

**Why the Correct Answer is Right**
Vertical transmission in the third trimester can occur through various mechanisms, including transplacental passage of pathogens or congenitally acquired infections. However, some diseases are not transmitted vertically in the third trimester, as they are acquired during delivery or have a different mode of transmission.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a disease that is typically transmitted vertically in the third trimester.
**Option B:** This option is incorrect because it is not a disease that is typically transmitted vertically in the third trimester.
**Option C:** This option is incorrect because it is not a disease that is typically transmitted vertically in the third trimester.

**Clinical Pearl / High-Yield Fact**
To determine the mode of transmission of a disease, consider the timing of infection, the route of exposure, and the pathophysiology of the disease. This will help you identify whether a disease is transmitted vertically, horizontally, or through other means.

**Correct Answer: C. Rubella**