"Trick" question?

[Grounded]

Another thread made me think of this question that I recently got wrong- Courtesy of NAUI:

What causes decompression sickness?
a) Nitrogen
b) Ascending too quickly
c) staying down too long
d) dehydration

I chose "B." My reasoning was that a) everyone absorbs nitrogen on a dive, but not everyone gets bent on every dive; c) you can stay down as long as you want, so long as you have enough gas to make to make the decompression stops (recognizing that all tables and decompression models are experimental and that someone may still get and "undeserved hit"); d) dehydration is said to increase the likelihood of a hit, but doesn't cause it. But b) ascending too quickly is what allows the bubbles to come out of solution (like the soda bottle from the open-water class).

The correct answer was "A"

Did I just over-think this one? Or would anyone else have been tricked by this?

 

[do it easy]

Yeah, that question is a bit limited- you can get bent on heliox by ascending too quickly. You can also get bent on oxygen!

I think the question is couched in a "recreational" background where ONLY nitrogen is tracked to avoid DCS.

 

[jefffalcone]

It's a poorly worded question. Don't worry about it. I think they do things like that on purpose so that everyone gets one or two questions wrong.

Your reasoning is sound. Just nitrogen on it's one certainly does not cause DCS or everyone would be bent all the time. DCS can only be caused by a combination of factors, so all of the answers are wrong really.

I think I remember thinking that was a foolish question when I took my OW cert. Padi had the exact same poorly worded question.

 

[GSDMan]

Did I just over-think this one?

Yep. I do it all the time and I have to go back and re-read the question several times. DCS is caused by gases coming out of solution in the blood and tissues and the only answer that fits that definition is A. The question you answered is, "What can cause the gases to come out of solution?"

Or would anyone else have been tricked by this?

If I wasn't very careful, I would probably have given the same answer you did.

On my OW test, I answered a DIN question wrong. I was thinking DIN is the one with the threads, and all, and somehow thought the 1st stage WAS the valve. So I, quite incorrectly, said the seal stayed with the valve. I just stopped paying attention for a second and it cost me a 100 on my test.

Jim

 

[Jeff Toorish]

Yeah, that question is a bit limited- you can get bent on heliox by ascending too quickly. You can also get bent on oxygen!

I think the question is couched in a "recreational" background where ONLY nitrogen is tracked to avoid DCS.

Well put.

Jeff

 

[Diver Dennis]

Yeah, that question is a bit limited- you can get bent on heliox by ascending too quickly. You can also get bent on oxygen!

I think the question is couched in a "recreational" background where ONLY nitrogen is tracked to avoid DCS.

I think this is the correct answer. Nitrogen by itself does not cause DCS, it's nitrogen or any gas coming out of solution as bubbles.

TDI Decompression Procedures describes DCS as "A series of maladies associated with nitrogen or other gas coming out of the solution of the blood or tissues, sometimes as bubbles."

 

[Diver Dennis]

Yep. I do it all the time and I have to go back and re-read the question several times. DCS is caused by gases coming out of solution in the blood and tissues and the only answer that fits that definition is A. The question you answered is, "What can cause the gases to come out of solution?"


Jim

Gases coming out of solution is DCS. Nitrogen just happens to be the gas in most cases.

 

[Colliam7]

Another thread made me think of this question that I recently got wrong- Courtesy of NAUI: ... Did I just over-think this one? Or would anyone else have been tricked by this?

I had to laugh. We often refer to 'PADI questions', those questions where at least 2 (maybe more) of the answers are technically correct, but where there is ONE and only ONE answer that PADI wants, and it may not even appear to be correct, or the 'most correct'. Interesting to see that poorly worded exam questions are not the exclusive domain of any one agency..

 

[Diver Dennis]

I had to laugh. We often refer to 'PADI questions', those questions where at least 2 (maybe more) of the answers are technically correct, but where there is ONE and only ONE answer that PADI wants, and it may not even appear to be correct, or the 'most correct'. Interesting to see that poorly worded exam questions are not the exclusive domain of any one agency..

I agree and the scary part is that this is pretty important stuff, Nitrogen loading and DCS...

 

[ClayJar]

In the type of "recreational diving" covered by the class, nitrogen loading must exist for a DCS hit to take place. You cannot get DCS without ongassing, however, nitrogen loading need not result in DCS. It is necessary but not sufficient to trigger a DCS hit. (If you are pedantic, you can then consider "nitrogen" necessary but not sufficient for there to exist "nitrogen loading". :biggrin

Ascending too quickly may not result in DCS. (For example, if you do a single dive on which you plummet like a rock to 100' and then immediately rocket to the surface, you've not had much of any time to ongas, eh?) With a slow enough ascent, any amount of ongassing can be handled without DCS, therefore ascending too quickly is necessary but not sufficient to cause DCS.

Staying down too long may not result in DCS. If you define "too long" as "in excess of the tables' NDL", you just have to ascend less quickly (i.e. stops and what have you). However, if you define "too long" as meaning a dive profile such that the gas remaining is insufficient for the ascent required to offgas the nitrogen loading acquired by diving said profile, then staying down "too long" could indeed be the root cause of a DCS hit, as it would cause too quick an ascent. Taking that second definition, staying too long may be enough to trigger a DCS hit; however, as you can *also* trigger a DCS hit by ascending too quickly while you still have plenty of gas remaining, it is sufficient but not necessary.

Dehydration may increase the probability that any given profile may result in DCS, but it alone is also not a cause. It is neither necessary nor sufficient to trigger a DCS hit.


The question was "What causes decompression sickness?" Such a question can rightly be interpreted as "What is necessary and sufficient to cause decompression sickness?" Of the four listed answers to the question as asked, none are correct. The proper response to such a question, then, is to mark your explanation on the answer sheet and then discuss with your instructor the logic of the question and each answer, explaining why the problem is broken. Assuming your instructor values knowledge and consideration, he should judge your answer to be more correct than the given responses and therefore credit you for a proper response to the question. (I don't know if anyone else allows that, but a NAUI instructor can certainly agree with you in a dispute with the test or key, provided you make the case.)

I seem to routinely take issue with test questions such as this one. My personal favorite are those questions where two answers can be taken as correct, especially if I can argue the position that my answer is actually *more* correct than the answer claimed to be correct. I am always polite, of course, and no discussion has ever had a material effect on passing or failing (I'd pass handily either way), but I love discussing "broken problems" on the test, as it turns multiple choice into essay questions, which are a much better gauge of acquired knowledge.

(Oh, and by the way, discuss it *before* they grade the exam, or if they'd rather grade first, be sure to note that you plan to discuss it. Otherwise, they'll just think you're trying to weasel a few more points instead of accepting that you disagree with a broken problem.)