**Core Concept**
Brain death is a clinical diagnosis characterized by the irreversible loss of all functions of the brain, including the brainstem. It is a critical concept in determining organ donation eligibility and withdrawal of life-sustaining therapies.

**Why the Correct Answer is Right**
In brain death, there is a loss of cerebral blood flow and metabolism, leading to the collapse of the blood-brain barrier and the release of various neurotransmitters and cytokines. The absence of brainstem reflexes, such as pupillary responses, corneal reflexes, and oculovestibular reflexes, confirms the diagnosis. Brain death is typically confirmed through a combination of clinical examination, imaging studies, and laboratory tests, including electroencephalography (EEG) and cerebral blood flow studies.

**Why Each Wrong Option is Incorrect**

**Option A:**
This option is not relevant to brain death. Apnea testing is a critical component of brain death determination, but it is not a characteristic of the condition itself.

**Option B:**
This option is incorrect because brain death does not involve the loss of spinal cord function. The spinal cord can remain intact and functional even in the presence of brain death.

**Option C:**
This option is incorrect because brain death does not involve the loss of cardiac function. Cardiac function can remain intact even in the presence of brain death, and patients with brain death can still exhibit cardiac contractions and blood pressure.

**Clinical Pearl / High-Yield Fact**
Brain death can be confirmed through a combination of clinical examination, imaging studies, and laboratory tests. The American Academy of Neurology recommends using a combination of apnea testing, clinical examination, and imaging studies to confirm brain death.

**Correct Answer: D. Brain death is characterized by the irreversible loss of all functions of the brain, including the brainstem.**