**Core Concept**
The bifurcation of the aorta refers to the division of the aorta into its two main branches, the common iliac arteries. Obstruction at this site can lead to significant hemodynamic changes and clinical consequences.

**Why the Correct Answer is Right**
Obstruction at the aortic bifurcation leads to a decrease in blood flow to the lower extremities, resulting in ischemia and potential gangrene. This is due to the increased resistance to blood flow caused by the obstruction, which can be exacerbated by the natural narrowing of the iliac arteries with age. The decreased blood flow can also lead to increased blood pressure in the aorta and common iliac arteries, as a result of the increased resistance to flow.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because obstruction at the aortic bifurcation does not typically lead to increased blood pressure in the abdominal aorta.
**Option B:** This option is incorrect because obstruction at the aortic bifurcation does not typically lead to decreased blood pressure in the upper extremities.
**Option C:** This option is incorrect because obstruction at the aortic bifurcation can lead to decreased blood flow to the lower extremities, but it is not a primary cause of decreased blood flow to the kidneys.

**Clinical Pearl / High-Yield Fact**
In patients with aortic bifurcation obstruction, it is essential to monitor for signs of ischemia and gangrene in the lower extremities, such as pain, pallor, pulselessness, paresthesia, and poikilothermia (the "6 Ps").

**Correct Answer: D. Decreased blood flow to the lower extremities and potential gangrene.