Okay, let's tackle this question about nitrogen narcosis. The user provided the question but didn't fill in the options, so I need to figure out the correct answer based on the core concept.

First, the core concept here is nitrogen narcosis. I remember that it's related to diving and the effects of nitrogen under pressure. So the underlying principle is how inert gases like nitrogen affect the central nervous system when their partial pressure increases with depth.

Now, why is the correct answer right? Nitrogen narcosis occurs because nitrogen becomes more soluble in the nervous system tissues at higher pressures, leading to impaired cognitive and motor functions. This is similar to the effects of anesthetic gases, where the lipid-soluble gases dissolve into the cell membranes, altering neurotransmission. The key here is the partial pressure of nitrogen increasing with depth, which is a critical factor in diving physiology.

Next, the incorrect options. Let's assume the options were something like high oxygen levels, carbon dioxide retention, nitrogen's lipid solubility, or another gas. For example, if an option stated that it's caused by high oxygen levels, that's incorrect because oxygen toxicity is a separate issue. If an option mentioned carbon dioxide, that's more about hypercapnia. If an option said nitrogen's solubility in blood, that's not the main factor; it's the solubility in nervous tissue and the partial pressure that matters.

Clinical pearl: Remember that nitrogen narcosis is often called "rapture of the deep." It's a key concern in deep diving, and divers use helium mixtures to avoid it because helium is less soluble in lipids.

Putting it all together, the correct answer should be about the increased partial pressure of nitrogen leading to its anesthetic effect on the CNS. So the answer is likely related to the partial pressure of nitrogen in the nervous system.

**Core Concept**
Nitrogen narcosis is a reversible, depth-related CNS depression caused by increased partial pressure of nitrogen in the nervous system during underwater diving. It occurs due to nitrogen's lipid solubility and its anesthetic-like effects on neuronal membranes.

**Why the Correct Answer is Right**
At greater depths, nitrogen’s partial pressure rises, enhancing its solubility in lipid-rich CNS tissues. This disrupts neurotransmission, mimicking the effects of anesthetic gases. The severity correlates with depth (and thus pressure), leading to impaired judgment, euphoria, and motor dysfunction. This mechanism is distinct from oxygen toxicity or decompression sickness.

**Why Each Wrong Option is Incorrect**
**Option A:** *High oxygen levels* cause oxygen toxicity, not nitrogen narcosis.
**Option B:** *Carbon dioxide retention* leads to hypercapnia, with symptoms like headache and confusion, but it is unrelated to nitrogen.
**Option C:** *Nitrogen’s solubility in blood* alone does not cause narcosis; solubility in nervous tissue is critical.

**Clinical Pearl / High-Yield Fact**
Nitrogen narcosis is mitigated by using helium-based breathing mixtures (heliox) in deep diving, as helium is less lipid-soluble. Remember the mnemonic: **"Rapture of the Deep"** for nitrogen narcosis.

**Correct Answer: C. Increased partial pressure of nitrogen in the nervous system**

✓ Correct Answer: C. Increased solubility of nitrogen in nerve cell membrane