**Core Concept**
Respiratory depression refers to a reduction in the rate or depth of breathing, often caused by the action of a substance on the central nervous system (CNS). This condition can be fatal if left untreated, as it can lead to hypoxia and death. The CNS regulates breathing through the medulla oblongata, which responds to changes in CO2 and O2 levels.

**Why the Correct Answer is Right**
The correct answer is the substance that does not cause respiratory depression. The primary mechanism of respiratory depression involves the inhibition of the medullary respiratory centers, which decreases the sensitivity to CO2 and increases the apneic threshold. This can lead to a decrease in the respiratory rate and depth. Substances that cause respiratory depression often act on the GABA_A receptor, which is an inhibitory neurotransmitter receptor that can lead to sedation and decreased breathing.

**Why Each Wrong Option is Incorrect**
**Option A:** Opioids are a classic example of substances that cause respiratory depression. They act on the mu opioid receptor, which activates the GABA_A receptor and leads to decreased breathing. This is why opioids are often used for pain relief, but can be dangerous in high doses.
**Option B:** Benzodiazepines also cause respiratory depression by acting on the GABA_A receptor. They increase the affinity of GABA for the receptor, leading to increased inhibitory neurotransmission and decreased breathing.
**Option C:** Barbiturates are another class of substances that can cause respiratory depression. They act on the GABA_A receptor, increasing the duration of the inhibitory postsynaptic potential (IPSP) and leading to decreased breathing.

**Clinical Pearl / High-Yield Fact**
The mnemonic "Opioids, Benzodiazepines, Barbiturates, and Alcohol" (OBBAs) can help you remember the common substances that cause respiratory depression. This is a classic exam trap, as students often forget that Opioids and Benzodiazepines are also responsible for respiratory depression.

**Correct Answer: D.