**Core Concept**
Ischemic necrosis of the renal cortex with relative sparing of the medulla is indicative of a condition that compromises blood flow to the renal cortex. This is often seen in conditions that lead to thrombotic microangiopathy or disseminated intravascular coagulation (DIC), which cause microvascular occlusion and subsequent tissue ischemia.

**Why the Correct Answer is Right**
Disseminated intravascular coagulation (DIC) is a condition characterized by the formation of blood clots within the microvasculature, leading to ischemia and necrosis of various organs, including the kidneys. In DIC, the formation of thrombi in small vessels can lead to renal cortical ischemia, resulting in necrosis. The relative sparing of the medulla is likely due to its more robust blood supply and higher tolerance to ischemia. The renal medulla receives blood from the interlobar arteries, which are larger and less prone to thrombosis compared to the cortical vessels.

**Why Each Wrong Option is Incorrect**
**Option B:** Multiple myeloma is a malignancy that can cause renal failure due to hypercalcemia, cast nephropathy, or amyloidosis. However, it does not typically cause ischemic necrosis of the renal cortex.

**Option C:** Polycystic kidney disease is a genetic disorder characterized by the development of cysts within the kidneys, leading to renal enlargement and eventual loss of renal function. While it can cause renal failure, it does not typically cause ischemic necrosis of the renal cortex.

**Option D:** Pyelonephritis is an infection of the renal parenchyma and collecting system, which can cause renal scarring and loss of renal function. However, it does not typically cause ischemic necrosis of the renal cortex.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that DIC can cause renal cortical necrosis, leading to acute anuria and uremia. This condition is often associated with trauma, sepsis, or malignancy, and prompt recognition and treatment are critical to prevent complications.

**✓ Correct Answer: A. Disseminated intravascular coagulation**