Question of the Week 2

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Here’s another example of a question from Askdoc USMLE Step 1 Prep Course Q Bank. We also post this Q&A in our Askdoc Facebook Fan Page. Try to practice answering the question before peeking into the answer.

equestrian

Question  # 2:

An eighteen year old member of the equestrian team was thrown off her horse. Fortunately she did not fracture her spine. Unfortunately, she has problems with moving her right arm aside from inability to abduct her shoulder and flex her elbow, she also cannot touch her opposite shoulder with her right hand. This inability to touch her left shoulder with her right hand indicates an injury in which of the following structure?

a.) Axillary nerve
b.) Musculocutaneous nerve
c.) Lateral Pectoral nerve
d.) Upper trunk of the brachial plexus
e.)Suprascapular nerve

 

*****

usmle q bank

Check out the Answer below:

a.) Axillary nerve is incorrect. The axillary Nerve supplies the deltoid and teres minor muscle which are responsible for abduction and lateral rotation.

b.) Musculocutaneus nerve is incorrect. The musculocutaneous nerve supplies the flexors of the forearm, namely the coracobrachialis, brachialis and biceps brachii.

c.) Lateral pectoral nerve is correct. The lateral pectoral nerve supplies the clavicular head of the pectoralis major muscle. The main muscle that allow you  to adduct and flex your arm and touch the opposite shoulder of your hand.

d.) Upper trunk of the brachial plexus is incorrect. Although injury to the lateral pectoral nerve can happen in an upper trunk injury, it is not always true. Upper trunk injury can occur with the lateral pectoral nerve intact. Therefore the inability to touch the opposite shoulder ith the affected hand only happens when the lateral pectoral nerve is specifically injured.

e.) Suprascapular nerve is incorrect. The suprascapular nerve supplies the supraspinatus and infraspinatus muscles which is responsible for abduction and lateral rotation respectively. The supraspinatus muscle is responsible for the first 15 degrees of abduction, while the infraspinatus is the pricipal lateral rotator of the arm.

*****

Unlike the straightforward facts-based question. This type of question is what we would call tough question in the USMLE. Actually the facts and questions being asked is simple and easy, but it’s the way the question is asked that makes it hard.

Half of the questions you will encounter in the USMLE are straightforward facts-based question like the first question in the question of the week. If you know the facts you can answer the questions. You can open the textbook, google for the answer and you will get it right.

About one fourth of the questions require you to do some analysis first before you can answer the question. Even if you know all the facts, you can still get the answer wrong. However, given enough time and knowing all the facts, most everyone can answer this type of question correctly. But of course, being the USMLE is a time-limited exam, you won’t have enough time and so a lot of people will get this kind of questions wrong. There is a systematic method of studying for these type of questions, ensuring that you will be able to answer those questions in the limited time given and they are outlined in my book ‘How to Master the USMLE Step 1’ and taught to my students in the course.

Another one-fourth of the questions are deliberately made harder by the test-makers by employing certain techniques that tempts you to choose the wrong answers. Distractors and two-step thinking you may say. They are part of them, but only a small part of them and probably the least effective ones. The most effective techniques makes use of your thought pattern, the way you studied, the way you work through questions to trick you into choosing the wrong answer even if you know the right one. Again there are methods to work through these type of questions so you get the right answer and they are outlined in my book and taught in my course.

Question number two of the question of the week employs the last two techniques to make this question a tough question. We will discuss and study them.

In most cases, people instinctively will choose D as the right answer. D is the correct diagnosis but the wrong answer. Instinctively, when we encounter a series of signs and symptoms, we immediately begin to diagnose the case. A lot of times we presume the question will require us to diagnose the case and so long as the test maker deliberately places the correct diagnosis in the choice, most people will choose just that. However, the actual question is asking what lesion is causing the patient’s inability to touch his opposite shoulder and not the diagnosis.

I call this technique ‘forcing people to answer the question NOT asked’ and this is just one technique. There are lots more discussed on my book and taught in my course. There are also many variant of each of these techniques. This question came from the anatomy qbank in my course. And all these various techniques are used through out all the different subject qbank in the course. And for those in the moderated prep course, they are discussed in the chat sessions and taught how to avoid these traps and anticipate for them in their review.

Suggestions on how to counter this range from, not diagnosing until you have read the question to reading the question first before reading the case. Both are not the best solution. There is a reason we start diagnosing the case when we start reading and it is because it is very efficient, we save time and it works most of the time. In more than half the questions, you really need to diagnose the case and most will be straightforward facts based questions and this method is pretty efficient in dealing with them. The other method will add from 5 to 30 seconds more to the time needed to answer the question and so not more efficient. What you need to do is to make sure to take the extra step of reading the question and making sure what the question being asked really is before proceeding. This will add at most 2 to 3 seconds to the answering process.

Even though the answers given by most in the post is B, the first answer chosen was D, which was corrected to B hours later. The reason others chose B was because they had seen someone answer B which made them aware that D might not be the right answer, so they reread the question, think through it longer and decided to go with B. In the actual exam, you can’t see other people’s answer and you have just over a minute to read and answer the question. So there is no time to reread it and think things through. In the course, where students have only a minute to answer the questions, almost everyone chose D.

In the actual exam, most will choose D, think this question is easy, tell their friends that USMLE is easy and get the shock of their lives when they find out they have failed or got a really low scores. Those who are astute enough to avoid this trap, will discover they need to do a difficult analysis to get the right answer, think USMLE is hard, tell their friends USMLE is hard, worry if they will pass and get surprised that they got a good score.

Now that you know D is not the right answer, you will now have to decide which of the individual nerves is the right answer. You now need to do an analysis. There are two ways to approach answering this question and depending on how you approach it whether the analytical process is easy or not. The first approach is how most of you will do it, since you studied anatomy in this context and that’s how it is presented in textbooks and notes. And if you use this approach, you will be left with two possible answers with a bigger chance you will chose the wrong one which most of you did. Using the second method, which you will know how to use if you took the time to study anatomy in another context, you will still be down to the same two answer choices but with a bigger chance of getting the right answer.

In order to touch the opposite shoulder you need to be able to bring your hand across the chest and raise your hand to touch your shoulder. So you will think pectoralis major to bring the hand across the chest and biceps to to raise the hand. This is how you will think because you memorized anatomy by muscle, origin, insertion, action and innervation. So you use muscle as the basis for analysis. Most test makers knows this and will deliberately use it to throw you down the wrong path. So you are down to lateral pectoral nerve and musculocutaneous. If there was a choice for both nerves, you would probably chose that as the correct answer but there isn’t. But you have to break the tie and if you are unable to do that, you have to guess, and it seems most of you guess it’s biceps. Of course that is not the answer.

Now the other approach required you to study group actions across a joint and group innervation. By using group actions as the basis for analysis it is easier to see which answer is better. The group action needed to touch the opposite shoulder is as follows, flexion at the elbow joint and flexion, adduction and internal rotation at the shoulder joint. Elbow flexors include the biceps, brachialis and coracobrachialis innervated by the musculocutaneous and the brachioradialis, innervated by the radial. In the shoulder, while there are multiple muscles that flex, adduct and internally rotate the joint, only one muscle does all three and that is the pectoralis major. If that is lost, the only way the person will be able to do all three movements in a coordinated manner is by rehab so the other muscles can effectively coordinate with each other to accomplish the movement. Meanwhile even without the musculocutaneous nerve, elbow can still be flexed by the brachioradialis especially if the hand is in a semi pronated manner. Therefore, lateral pectoral nerve is the best choice. You need to study both ways to answer this question with ease and my students are told to study anatomy both ways. It is heartening that all my students chose lateral pectoral nerve and not musculocutaneous.

Therefore, a lot of times, whether you can easily analyze a question or not depends on how you studied the topic concerned. Too often some of my students will insist they don’t have to study the topic the way I have outlined since they can remember the topics just fine studying it their way. They don’t realize that come time to answer questions, they will take two to five times longer to come up with the correct answer or sometimes not at all compared to people who studied it In the context I have prescribed. Remember, the questions were deliberately designed to make it harder for you to chose the right answer. Test makers take advantage of your thought process and the way you usually studied to make the questions harder to answer. And you can counter them if you know them. These are explained in my book, albeit not as detailed and thorough as in the moderated prep course.

Of course, you could have answered this question without much analysis if you are a neurologist, or you encountered cases of Erb-Duchenne palsy in your neurology rotation. You would know that not all cases of Erb-Duchenne palsy involve the lateral pectoral nerve and the test to determine if there is lateral pectoral nerve involvement is to ask the patient to touch his opposite shoulder.

The third point you have to take note of is this particular fact. The instruction on the exam is to choose the best answer, not the correct answer. Therefore there can be more than one correct answer in the choices but only one best answer. If lateral pectoral nerve was not in the choices presented, then musculocutaneous would have been the best answer.

Another example would be a question about action of the biceps. If the choices were a. supination, b. flexion, c. supination and flexion. All choices will be correct but c is the best answer.

Again, a lot of people miss the point. In the exam, they chose a correct answer, not realizing there are two to three of them, but not the best answer. Consider the question easy and not even flag it for review. Tell all their friends USMLE is easy and get the shock of their lives when they fail or get a low score.

Meanwhile their more astute colleagues was able to catch the other correct answers, had to spend time determining the best answer. Flag this question as hard, tell their friends that USMLE is hard and pleasantly surprised that they have scored well or even aced this exam since they were astute enough to catch these tricks and therefore have an infinitely better chance at getting the right answer then their other colleagues.

Most people when practicing online q banks take note only of the explanations why the answer is right, presuming that somehow their lack of knowledge is why they can’t get the right answer. Very few take the time to analyze why they got an answer wrong even if they knew the facts. They don’t realize that they were tricked into choosing the wrong answer and that only by analyzing their thought process and thinking through the reasons why they chose the wrong answer can they correct the problem and get higher scores. This is why this important step is discussed in my book How to Master the USMLE Step 1 available at Amazon.com and covered in my course.

Copyright © Askdoc. All rights reserved. All questions appearing in the Question & Answer are part of the Askdoc Prep Q Bank.

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