**Core Concept**
Brain death is defined as the irreversible loss of all functions of the brain, including the brainstem. This concept is crucial in determining death in patients who are on life support. The diagnosis of brain death involves a series of clinical tests.

**Why the Correct Answer is Right**
The correct answer is related to the loss of brainstem function, as the brainstem controls vital functions such as breathing, heart rate, and blood pressure. The brainstem also regulates the body's automatic functions, including those necessary for survival. Loss of brainstem function indicates that the brain is no longer able to sustain life.

**Why Each Wrong Option is Incorrect**
**Option A:** Loss of cortical function alone is not sufficient to declare brain death, as some brainstem functions may still be intact.
**Option C:** Spinal reflexes can persist even after brain death has occurred, as they do not require brain function.
**Option D:** Corneal reflex is a brainstem-mediated reflex, but its absence alone does not confirm brain death.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that brain death is diagnosed by demonstrating the absence of all brainstem reflexes, including pupillary, corneal, and oculovestibular reflexes, as well as the absence of respiratory effort.

**Correct Answer:** B. Brain stem function