**Core Concept**
Hyperbaric oxygen therapy (HBOT) involves inhaling pure oxygen in a pressurized room or through a tube. It increases oxygen levels in the blood, which can aid in wound healing, reduce swelling, and treat various medical conditions. However, HBOT's effectiveness is limited by its inability to cross the blood-brain barrier.

**Why the Correct Answer is Right**
The correct answer is because HBOT is not useful in treating conditions that require oxygen to cross the blood-brain barrier. This includes conditions like acute ischemic stroke, where oxygen is needed to salvage brain tissue. Since HBOT cannot increase oxygen levels in the brain, it is not effective in treating acute ischemic stroke.

**Why Each Wrong Option is Incorrect**
* **Option A:** HBOT is not useful in treating conditions that involve carbon monoxide poisoning. This is incorrect because HBOT is actually effective in treating carbon monoxide poisoning by helping to displace carbon monoxide from hemoglobin and increase oxygen delivery to tissues.
* **Option B:** HBOT is not useful in treating conditions that involve gas gangrene. This is incorrect because HBOT is actually effective in treating gas gangrene by reducing the growth of anaerobic bacteria and promoting wound healing.
* **Option C:** HBOT is not useful in treating conditions that involve carbon monoxide poisoning or gas gangrene. This is incorrect because, as mentioned earlier, HBOT is effective in treating both conditions.

**Clinical Pearl / High-Yield Fact**
HBOT is not a substitute for standard medical treatment, but it can be used as an adjunct therapy to enhance the effectiveness of other treatments. It's essential to carefully evaluate the indications and contraindications for HBOT before using it in clinical practice.

**Correct Answer:** D.