 In this video, we'll be talking through a question from Respiratory Physiology for USMLE Step 1. You're going to get an inside look on how an experienced test taker approaches a difficult Step 1 question. You'll be able to follow along as I think out loud and point out important elements of this Respiratory question. This is going to be great for those of you who are looking to sharpen your test taking skills or get an extra edge when it comes to studying for Step 1. So let's jump into the question. A 61-year-old man presents to the emergency department in respiratory distress. He's not seen a physician since he was a teenager, but he informs you that he's been experiencing progressive shortness of breath over the past several months. Okay, so the differential for respiratory distress is pretty broad, but the fact that he hasn't seen a physician since he was a teenager might be hinting to us that this is some kind of condition that's due to a missed screening appointment or something very chronic that was going on for a long time and wasn't caught because he wasn't seeing the physician. Recently, he becomes short of breath even walking to his mailbox. Denies using tobacco, alcohol, or illicit drugs. Okay, this is pretty important. The fact that he doesn't use tobacco means he's an auto-smoker. So things like COPD, specifically chronic bronchitis or emphysema, are probably less likely here. His temperature is 99.1, blood pressure 130 over 92, pulse 110, respiration is 24. Okay, so we notice right off the bat that he's tachycardic and tachypnec, but he doesn't have a fever. So potential causes of respiratory distress like pneumonia that are caused by an infection will probably be less likely in the absence of a fever. On physical exam, increased work of breathing and accessory muscle use, as well as cyanosis of his distal extremities. Okay, so we don't know quite yet where this question's heading, but if you ever see in a question stem something like increased work of breathing or accessory muscle use, it's trying to tell you that this is a very severe case of respiratory distress. This is a critically ill patient and we have to treat them as such. Okay, fine crackles are noted in all lung fields. Okay, when I hear things like fine crackles in a physical exam finding, that makes me think of certain differential diagnoses. One big one that you'll see a lot, especially on step one, is interstitial lung disease. Condition worsens and he's intubated for mechanical ventilation. The readout on the ventilator shows you that there is a lower than normal rise in lung volume for any given pressure. Okay, so from studying physiology, I know that the rise in lung volume for any given amount of pressure is also known as compliance. So if it's lower than normal, we're talking about a condition that causes decreased compliance. Which of the following is the most likely diagnosis in this patient? So let's look at the answer choices. Starting with A, APA1 antitripsin deficiency. If you're familiar with this condition, you know that it can cause a type of emphysema, but emphysema is a type of chronic obstructive pulmonary disease. And we know that COPD causes an increase in compliance. Whereas in this question, we're looking for something that causes a decrease in compliance. So I'm going to think that alpha 1 antitripsin deficiency is not the answer here. Furthermore, questions about alpha 1 antitripsin deficiency often have other features that are included in the syndrome of alpha 1 antitripsin deficiency. For example, liver findings like cirrhosis or fibrosis are very common. B, bacterial pneumonia. Again, like we mentioned when we were reading through the question, the patient's A-febrile, this patient doesn't really have any of the classic symptoms we would associate with bacterial pneumonia. Like a cough, especially fever chills, we would expect to see those. So I'm going to go ahead and say that's not the answer. C, chronic bronchitis. As we mentioned with answer choice A, chronic bronchitis is a type of COPD, and COPD causes an increase in compliance. So we know that for that reason, it can't be the answer. Furthermore, the patient, as we read in the question, doesn't use tobacco, and chronic bronchitis is a disease that it's associated with a prolonged history of smoking. So without tobacco use, it's very unlikely that the patient has chronic bronchitis. D, idiopathic pulmonary fibrosis. OK, this is an answer choice I would strongly consider. Because as we mentioned in the question, the patient has fine crackles on physical exam. And when we look past and see that he's got this progressive shortness of breath that's been evolving, those kind of fit the mold of an interstitial lung disease like idiopathic pulmonary fibrosis. Furthermore, idiopathic pulmonary fibrosis is a type of restrictive lung disease, which would cause a decrease in compliance as opposed to things like alpha 1 antitripsin deficiency and chronic bronchitis, which would cause an increase in compliance. So I'm going to think that D is probably the answer here. So for good measure, let's look at answer choice E, pulmonary tuberculosis. I'm also going to think that this is not the answer choice. Questions about tuberculosis generally give you some hints that the patient's been exposed to TB. For example, he's been to prison. He's been to a country where TB is endemic. He just immigrated to the country from one of those areas. So I'm going to go ahead and say the answer here is D, idiopathic pulmonary fibrosis. So now that we know the answer is D, idiopathic pulmonary fibrosis, let's do a quick review on what lung compliance is. So if you take a look at these graphs here, you'll see that you have transpulmonary pressure on the x-axis and lung volume on the y-axis. And these curves here represent compliance of different types of lungs. So if we look at the green one here, we'll call that a normal compliance. Red line here is going to signify an increase in compliance, and the blue line is going to be a decrease in compliance. So what causes an increase in compliance? If we remember our formula for compliance, it's change in volume over change in pressure. So for a given amount of pressure that's introduced into the lung, how is the volume going to change? So things that cause increased compliance are things like COPD because it causes air trapping. Whereas what causes decreased compliance, that's things like interstitial lung disease. So restrictive lung diseases prevent the lung from expanding properly and can cause a decrease in compliance. So you can force more pressure in, but the amount of change in volume is not going to be the same as in a normal lung. So I hope you enjoyed this video about pulmonary physiology. I think this is a really good question, and you probably noticed if you're one of those test takers that jumps to the bottom of the question and reads that portion before looking at the whole thing, that you may be able to answer this without reading the entire question stem. But I think it's worth it to look at the entire question in this case because the history and the physical exam in particular give us some really important details that will help us answer the question which is asking for a diagnosis in this case. So even if you didn't know about compliance and the fact that idiopathic pulmonary fibrosis causes a decrease in compliance, you might be able to pick up on those physical exam findings. I know this is a type of interstitial lung disease.