**Core Concept:** Vitamins play essential roles in various physiological processes, and excess intake of certain vitamins can lead to toxicity and adverse effects, including birth defects (teratogenesis). Teratogenic substances disrupt normal fetal development, leading to congenital anomalies.

**Why the Correct Answer is Right:** Vitamin A, when taken in large doses, can cause hypervitaminosis A. This condition leads to toxicity, which may result in teratogenesis. Excess vitamin A can induce morphological changes in the fetus, potentially causing birth defects.

**Why Each Wrong Option is Incorrect:**
A. Vitamin D: While excess vitamin D can cause hypervitaminosis D, it is not teratogenic.
B. Vitamin E: Large doses of vitamin E can cause toxicity, but it is not considered teratogenic like vitamin A.
C. Vitamin K: Vitamin K deficiency is more concerning, and excess intake of vitamin K is not known to cause teratogenesis.
D. Vitamin C: While excessive vitamin C intake can lead to toxicity, it is not considered teratogenic like vitamin A.

**Clinical Pearl / High-Yield Fact:** Adequate vitamin A intake is important during pregnancy, as a deficiency can lead to maternal and fetal complications. However, avoiding excessive intake is crucial to minimize the risk of teratogenesis.

**Correct Answer:** D. Vitamin A