**Core Concept**
Brain death is a clinical diagnosis that involves the irreversible loss of all functions of the brain, including the brainstem. It is a critical concept in forensic medicine, as it has significant implications for organ donation and the determination of death.

**Why the Correct Answer is Right**
The correct answer, **D. Heart rate un-responsive to atropine**, is not a feature of brain death because the heart rate response to atropine is mediated by the peripheral nervous system, specifically the autonomic nervous system. In brain death, the brainstem, which regulates autonomic functions, is damaged, but the peripheral nervous system, including the autonomic nervous system, may still be functional. Therefore, a person who is brain dead may still have a heart rate response to atropine. In contrast, features of brain death include complete apnea (A), absent pupillary reflex (B), and absence of deep tendon reflexes (C), as these are all mediated by the brainstem.

**Why Each Wrong Option is Incorrect**
**Option A:** Complete apnea is a feature of brain death, as it indicates the loss of respiratory drive from the brainstem.
**Option B:** Absent pupillary reflex is a feature of brain death, as it indicates the loss of brainstem function.
**Option C:** Absence of deep tendon reflexes is a feature of brain death, as it indicates the loss of spinal cord function, which is mediated by the brainstem.

**Clinical Pearl / High-Yield Fact**
In forensic medicine, brain death is often determined by a combination of clinical and radiological findings, including apnea testing, pupillary reflex testing, and imaging studies to rule out other causes of coma.

**✓ Correct Answer: D. Heart rate un-responsive to atropine**