Recognizing Zebras in Clinical Vignettes

Last updated on August 9th, 2023

The medical boards pride themselves on presenting multiple-choice questions that are clinically-relevant and fair. In other words, the boards are not trying to trick you. This means that most clinical vignettes will describe patients with a common form of a disease or disorder. After all, these are the cases most of us physicians spend our days assessing and treating. However, as clinicians we do need to recognize the rare condition. So the question is, how do you distinguish whether the question is about a “horse” or a “zebra?”

If it’s a “zebra,” there will be some information provided within the clinical vignette about the patient’s demographics that is unusual for that disease state, such as it being an unusual occurrence at that patient’s age, in their gender, or perhaps in their racial or ethnic group. For instance, if on the family medicine boards you’re presented with a classic presentation of acute cholelithiasis occurring in an adolescent who has never been pregnant, the patient’s age should alert you to etiologies that are not common in adults but common in children and adolescents, such as presence of hemolytic disease which, when present, leads to formation of black pigment stones. Or if on the psychiatry boards, you have a dementia presentation described in a 50 year old man without a family dementia history, the etiology is probably not Alzheimer’s disease (unless the patient has trisomy 21, perhaps of a mosaic form which has less evident physical stigmata of Down’s syndrome). Or if the vignette presents a young East-Asian woman who presents to the ER unconscious, you should wonder why her race or ethnicity is described in the vignette. Let’s pretend the clinical vignette states that you learn from family members that she immigrated from a rural province of China and has been severely despondent about her move to this country. (See discussion of organophosphate poisoning below).

Or perhaps the question suggests a “zebra” because it includes a piece of information about the clinical presentation that is clearly out of place. An example would be a patient with nocturnal panic attacks – which are not uncommon – who also reports that he’s already taking some unnamed medication given him by his primary care physician “for anxiety.” The test-taker’s initial impression may be that the current treatment is not working. This indeed may be the case. An alternative explanation is that the patient has been prescribed a short-acting medication whose effects wear off prior to the next dose. A common culprit of nocturnal anxiety or panic is immediate release alprazolam. Although this medication has an FDA indication for panic disorder, it has a very short half-life and often leads to drug-withdrawal anxiety when medication plasma levels fall. The fact that the clinical vignette provides information about the occurrence of nocturnal attacks together with the patient taking “some medication” may be enough to trigger you to think beyond your first clinical hypothesis.

A third type of clue for a “zebra” relates to the nature or setting of the disease. Perhaps the question vignette mentions that the patient has traveled internationally (especially to a tropical country), or they’ve been involved in an unusual activity or hobby (for example, high altitude mountain climbing), or their occupation is described. In the latter case, pay attention to information suggesting the patient works in a factory or in an agricultural or rural setting. For example, exposure to organophosphates is much more common in individuals living in a farming or rural community because many herbicides are organophosphate compounds and are readily available. Recall that organophosphates are cholinesterase inhibitors and poisoning with one of these compounds leads to muscarinic and nicotinic overactivity and high rates of death from diaphragmatic failure. Intentional poisoning is the most common method of suicide in young women living in rural China. Accidental poisoning is also common. The example mentioned in the previous section – the young woman who emigrated from China, given her rural background, may have been knowledgeable about the lethality of these commonly obtained chemicals. Also, the fact that the clinical vignette discussed her despondent state of mind was also an important clue.

The motto for board exams is “Nothing in the vignette has been placed there by accident.” Every fact or other piece of information inserted into questions is designed to lead you down one diagnostic path or another, or down one treatment path or another. The information is there to help you make diagnostic and/or treatment distinctions.

Example Exam Question from Psychiatry Boards

Question: A 46 year old man returned last week to the US from a business trip to Japan. After arriving home he feels short of breath, tired, and “panicky.” When the feelings do not subside he takes a cab to the nearest emergency room. Upon questioning he tells the physician that he had a history of anxiety when younger but has for the most part overcome it. Which of the following disorders is this patient most likely to suffer from?

1. 1. Hyperthyroidism
   2. Pulmonary embolism
   3. Panic disorder
   4. Generalized anxiety disorder
   5. Post-traumatic stress disorder

Question Explanation

Pulmonary emboli are not common. There are dozens of individuals with anxiety disorders for every one person who develops a pulmonary embolism. However, in this case the information – not very subtle and therefore not that challenging – points to a medical cause for this man’s anxiety. First, he was sitting for 10-14 hours on a cramped flight, his symptom onset was abrupt, and he had not experienced significant anxiety for many years prior to this event.  Deep venous thrombosis, with pulmonary embolism as a sequela, is a well-known hazard of immobility on long airplane flights. Also, there is no supportive evidence provided in the clinical vignette suggesting post-traumatic stress disorder, generalized anxiety disorder or hyperthyroidism. Therefore, the two top choices are panic disorder and pulmonary embolism. Given the context of travel and no other symptoms suggesting panic disorder, pulmonary embolism is the best option.

If you need help preparing to take your boards you can access our board review courses for psychiatry and neurology at Beat the Boards, and all other medical specialities at The Pass Machine.