This gives us a concrete reason that confusing the two conditions would be especially dangerous, and we can verify whether it's necessary using negation. If (A) were false, and the treatments for OMS and CE were the same, then what happens to the argument? It falls apart, because mistaking CE for OMS wouldn't lead to any additional risk: it would be treated correctly.

The author's claim is that CE is more dangerous at higher altitudes, because presumably it would be less likely to receive correct treatment. That's why we need to assume (A), to affirm that a confusion with OMS would actually lead to the wrong treatment.

We already know that CE can quickly be life-threatening without treatment, so we don't need another reason that it's a dangerous condition. Even if coma were not one of its symptoms, that wouldn't harm the argument, so we can tell that (B) isn't necessary.

Like (B), (C) addresses the specific symptoms of one of these illnesses. Also like (B), (C) is not necessary—exactly how OMS and CE affect people aren't so important compared to their similarity of symptoms and the overall danger of CE.

Negating (C) might make OMS more dangerous, but that still doesn't affect the argument, because the conclusion is just about the relative danger of CE at different altitudes. How dangerous OMS could be doesn't change that.

(D) presents us with a strengthening answer that is not truly necessary—watch out for this kind of trap!

Although (D) strengthens by telling us that people's judgment may suffer at high altitudes, which could reasonably lead to poorer treatment of CE, the argument doesn't absolutely need this to be true. Even if people's judgment remains the same, CE could still be particularly dangerous at high altitudes if confused with OMS. Negating (D) doesn't wreck the argument, so it's not necessary.

It might be tempting to take (E) to mean that people experiencing symptoms of OMS simply wouldn't treat their condition, but that's not what (E) says. And even if it were, that would be a strengthening answer that was still not necessary to the argument.

Avoiding treatment for OMS is one way to make CE more dangerous, but it doesn't have to be the case: even if OMS is always treated promptly, applying the wrong treatment to CE would still count as extra danger. Compared (A), you can see this difference: (A) must be true, while (E) could be false.