26-year-old (male) with Lower Leg Pain

00:01 Okay, guys. Super important question here.

00:04 Outrageously clinically relevant.

00:06 Let's jump right in.

00:08 A 26-year-old male graduate student presents to the emergency department with a two-day history of worsening right lower leg pain.

00:19 He states that he believes his right leg is swollen compared to his left.

00:24 His only medical problem is generalized anxiety disorder which is managed effectively with psychotherapy.

00:32 He additionally smokes a pack of cigarettes daily but denies alcohol and illicit drug use.

00:39 His father died of a pulmonary embolism at the age of 43.

00:45 On physical examination, the right lower leg is warmer than the left, and dorsiflexion of the right foot produces pain.

00:54 His vital signs are 36.7 degrees Celsius.

00:58 Blood pressure is 126/74. Heart rate is 74 and the respiratory rate is 14.

01:07 What condition is most likely responsible for this patient's presentation? Answer choice A: Factor XI deficiency; Answer choice B: Factor V Leiden; Answer choice C: Von Willebrand disease; Answer choice D: Vitamin K deficiency; or Answer choice E: Hemophilia A Take a moment to come to the answer by yourself before we go through it together.

01:43 Okay, this is a very important question.

01:47 Let's think about the question characteristics first.

01:50 Well first thing, this is a hematology question within the category of internal medicine.

01:56 We're talking about someone coming in, they're having leg pain and all the answer choices somehow relate to the coagulation pathway or some type of thrombophilia so this is hematology.

02:07 This is a two-step question.

02:10 The first thing we need to do is try to figure out, based on the clinical presentation, what kind of clinical event is occurring and then second, we need to figure out what is the underlying process that's producing that clinical event.

02:26 Of course, in this case, stem is required because you really need to be able to go through the stem and pull out all the really important details to be able to come to bring the answer together.

02:39 So let's walk through it and first thing, let's determine what clinical situation is going on with this patient.

02:47 We have a young male coming in, 26-year-old male and his chief complaint is that he has leg pain.

02:55 The main differential diagnosis for having leg pain really includes injury and that can be in joints, bones, muscles, ligaments, tendons, or any soft tissue but also, leg pain can be caused by blood clots or problems with the lower spine.

03:15 Let's go through the characteristics of this patient's pain and we can use the Socrates mnemonic.

03:22 It's not absolutely necessary to have to do this. In an exam crunch, you may not want to do this and get around it but I think for the sake of our learning here as we're going through questions together, it's a useful exercise cuz you're gonna use it in other questions.

03:38 So, S the site. This guy's having pain in the right lower leg and he says it's in his calf.

03:45 Onset, well, it's been two days and it sounds like it's sudden with no real trigger.

03:51 We don't know character.

03:53 There is no comment regarding radiation.

03:55 Now associations, here he endorses swelling and you know, edema can be what we are then trying to think in our minds and then also, he mentions as an association warmth of the right leg as well.

04:09 Another association is that he has pain in that right foot when he has dorsiflexion of the foot.

04:17 For timing, we're assuming it's constant and things that make it better or worse or exacerbating and relieving factors, nothing generally makes it better and making it worse is dorsiflexion of the foot.

04:32 Severity, they don't tell us anything but obviously, it was bad enough to come to the emergency department.

04:38 The lack of back pain and lack of radiation of pain makes us think that it's probably not a lower spine pain and if you think about lower spine pain, things like sciatica or other types of leg pain, that usually manifests as upper leg pain.

04:59 While our patient's coming in with a lower leg/calf pain.

05:04 The sudden onset without an obvious trigger also makes us think less likely that it's injury that caused this pain.

05:14 There's something really important to the question stem.

05:17 We've discussed a lot of it, but we've learned he has a family history of pulmonary embolism that is probably trying to hint for us that he actually has a hereditary clotting disorder.

05:31 He mentions his dad had a pulmonary embolism at 43.

05:36 That's way too young.

05:37 Something has to be going on with that guy for his dad be having a PE at 43.

05:42 That's too young for that.

05:43 So that's the hint: hereditary cause.

05:45 We're also told that he smokes a pack of cigarettes a day.

05:50 Well, cigarettes are a hypercoagulable state condition which is another thing to consider.

05:56 Of all these things that we think about, a deep vein thrombosis or a deep vein clot is really the most likely cause of what's going on here and that's really supported by the fact that when he mentions in the question stem that there's dorsiflexion of the foot causing pain, that's classicly called Homan's sign which, just so you know, it's when you have the patient dorsiflex a foot and if they have a DVT, they're supposed to go, "ow, that really hurts." In reality, that never happens but on USMLE, they love it so just memorize it.

06:32 And they will ask you on your wards so memorize it for that as well.

06:35 So the diagnosis really here is a DVT.

06:37 Guy's coming in, right pain it hurts when he dorsiflexes, it's warmer, it's bigger, he's got a positive family history of a clotting disorder probably and he smokes.

06:46 Okay, he's probably got a DVT.

06:48 That's the easy part.

06:50 Now let's try to figure out what's the underlying mechanism of his DVT.

06:55 When you look at DVT's, the main cause of them usually is old age, surgery, a hereditary thrombophilia, or something causing a hypercoagulable state.

07:07 Here we have a young patient, not an old one.

07:10 He's got no history of surgery that we know of or any recent surgery but his father had a thrombotic event of the PE.

07:18 A hereditary thrombophilia is the most likely cause and it's probably exacerbated by the fact that he smokes.

07:27 So of all the answer choices, answer choice B, factor V Leiden is the correct answer and you can figure this out very quickly by knowing that that choice, factor V Leiden, will cause you to be hypercoagulable but every single other answer actually makes you likely to bleed so simply from that, you can figure out pretty quickly.

07:52 So let's go through the answer choices and explain why that is the case and how I figured that out so quickly.

07:59 So let's start with the right answer choice first, factor V Leiden, what this kid's got.

08:04 Factor V Leiden is the most common form of an inherited thrombophilia.

08:11 Thus if it's the most common form, you better believe you better memorize it and you better believe they're probably gonna ask you about it so get factor V Leiden in your head.

08:20 So most common form of inherited thrombophilia and it's most prevalent in people of European history.

08:26 This is an autosomal dominant condition and people who are homozygous for this condition are in at an even higher risk of having a thrombotic event.

08:37 It's caused by gene mutation that prevents factor V from being broken down by activated protein C.

08:46 As a result, because factor V is not being broken down, the clotting cascade will continue longer than usual which increases the risk of having a DVT.

08:59 When patients have a DVT, they will present with unilateral lower leg pain and swelling and they can often have, but not always, a positive Homan's sign which is in which they have pain with dorsiflexion of the foot.

09:15 Patients with factor V Leiden generally don't need to be on blood thinners chronically, unless they're at risk or they have other risk factors for blood clot and if they do, you should then manage the clot with putting the patient on a medication such as warfarin or one of the novel oral anticoagulants or in the case of this person's questions stem, we're also going to ask him to stop smoking to not exacerbate his clotting risk.

09:44 Now, let's go through the other answer choices and figure out why they're wrong.

09:49 Answer choice A, look at that. If you have a deficiency in factor XI, well factor XI, let's go ahead and take a look at your clotting cascade image.

10:00 Factor XI is part of the intrinsic pathway of blood clotting.

10:05 So if you have a factor XI deficiency, you have a deficiency in being able to clot and you're more likely to then bleed.

10:14 So look at your pathway right there, right in the middle of factor XI and if that doesn't work well, well, you can't even clot so factor XI deficiency would make you bleed, not clot.

10:24 Not going to be the answer to cause a DVT.

10:27 Answer choice B, factor V Leiden, we explained. Factor V hanging around more.

10:32 Again, go back to look at your clotting cascade. Factor V there, in addition with Xa, going to be converting X to Xa which will then convert prothrombin to thrombin which then leads ultimately to a clot so that's the correct answer because factor V Leiden is a hypercoagulable condition.

10:53 Answer choice C, Von Willebrand disease.

10:56 This is a deficiency in Von Willebrand factor and that's involved in clotting so if you have a deficiency of Von Willebrand factor, you're going to bleed, not clot.

11:08 Answer choice D is vitamin K deficiency. Very important.

11:14 This one you could have deleted within 2 nanoseconds because vitamin K deficiency is common in newborns.

11:22 That's where you're going to get the vitamin K question, not in an adult.

11:26 So vitamin K, it's in newborns and again, if you don't have vitamin K, you are likely to have a bleeding state, not a clotting state.

11:36 Answer choice E is hemophilia A.

11:39 Well, hemophilia A is a genetic defect in factor VIII.

11:44 The way to remember that is A and 8, they kind of sound the same, A and 8.

11:49 So factor VIII deficiency or defect is going to lead to an inability to properly clot so that's again another bleeding risk.

12:00 So that also will not be the right answer.

12:02 So now let's review some high-yield facts regarding DVT's and factor V Leiden as our closing points, now that you understand the condition and how you could have eliminated the right and wrong answers.

12:14 So talking about DVT's: first, so a DVT is a blood clot in a deep vein.

12:20 It most commonly works occur in the legs.

12:22 They can also happen in the arms, know that.

12:25 Put it in your head. But most commonly and again for questions, most common in the legs.

12:30 There's sudden onset, that's when the patient notices it and what they're going to notice and describe in the question stem for you is pain, swelling, redness or warmth in a certain limb.

12:42 The mechanism of how people get these deep vein clots is really explained by what's called Virchow's triad, which is the triad of venous stasis hypercoagulability and changes in the endothelial blood vessel wall that leads to a clot formation.

13:01 Now you can also have associated risk factors such as having an underlying hereditary thrombophilia such as factor V, or smoking as what this person was doing, and a pulmonary embolism really is why you care about DVT's.

13:18 So having a DVT, yeah it's uncomfortable but who cares? I'll tell you why we care. It's because of a PE.

13:25 The pulmonary embolism is a severe complication of a DVT because a blood clot in the leg can go up through the venous system, up to the right side of the heart and then go to the lung and shoot and cause a PE.

13:39 If it's big enough, that can be life-threatening.

13:41 Really here, people who have DVT's, we wanna prevent them by making them, you know, if they're sitting around, walking around, or if necessary, we have to anticoagulate them.

13:51 Let's review some high-yield facts regarding factor V Leiden.

13:55 Now, this is a hypercoagulability disorder.

13:58 That is excessive blood clotting and the hereditary condition here is that it's autosomal dominant and it's actually the most common hereditary hypercoagulability disorder so you better know it.

14:11 The mutation here is a gene coding for factor V in which, if you see factor V is involved in converting X to Xa in the common pathway, that will eventually lead to clot formation.

14:25 If you have a defect in factor V, protein C can't break it down so factor V hangs around and you get more clot formation.

14:34 Of course, the manifestation here in these patients is recurrent venous thrombosis that are, could be dangerous if they get a PE.

14:42 The treatment here is, again, preventing it and if necessary, you give anticoagulation.