**Core Concept**
The vessels of the umbilical cord are crucial for the exchange of oxygen, nutrients, and waste products between the fetus and the placenta during pregnancy. The umbilical cord contains two arteries and one vein, which are responsible for this exchange.

**Why the Correct Answer is Right**
The two arteries in the umbilical cord carry deoxygenated blood and waste products from the fetus to the placenta, where they are processed and excreted. The single vein returns oxygenated, nutrient-rich blood from the placenta to the fetus. This arrangement allows for efficient exchange of substances between the fetus and the placenta.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as there is no fourth vessel in the umbilical cord.
**Option B:** This option is incorrect as the umbilical cord does not contain a single artery, but rather two arteries.
**Option C:** This option is incorrect as the umbilical cord does not contain a single vein, but rather one vein.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the two arteries in the umbilical cord are responsible for carrying deoxygenated blood, while the single vein returns oxygenated blood. This knowledge is crucial for understanding fetal development and placental function.

**Correct Answer:** D. The umbilical cord contains two arteries and one vein.