**Core Concept**
Ethanol is a central nervous system depressant that can cause a range of effects, from mild intoxication to coma and death. The blood ethanol concentration (BEC) is a critical factor in determining the severity of ethanol's effects.

**Why the Correct Answer is Right**
The National Institute on Mental Health and Substance Abuse (NIMHSA) has established that a BEC of 300 mg/dL or higher is considered fatal. This is because ethanol can cause respiratory depression, hypotension, and cardiac arrest at high concentrations. Ethanol is metabolized by alcohol dehydrogenase, which converts it into acetaldehyde and then into acetate. The brain's sensitivity to ethanol is increased by the presence of gamma-aminobutyric acid (GABA) receptors, which are inhibited by ethanol.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the National Institute on Mental Health and Substance Abuse (NIMHSA) does not specify a BEC of 200 mg/dL as fatal.
**Option B:** This option is incorrect because a BEC of 250 mg/dL is generally associated with severe intoxication, not necessarily fatal levels.
**Option C:** This option is incorrect because a BEC of 350 mg/dL is significantly higher than the established fatal level of 300 mg/dL.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the BEC can fluctuate rapidly due to factors like gastric emptying, metabolism, and hydration status. Clinicians should consider these factors when interpreting BEC results to avoid underestimating or overestimating the severity of ethanol intoxication.

**Correct Answer: D. 300 mg/dL or higher.**