Last updated on June 7th, 2023
Medical exam boards take pride in clinically relevant and fair multiple-choice questions. This simply means the board is not trying to trick you. Most of the case vignettes describe a patient that has a common form of the disease. After all, we as physicians spend the majority of our time assessing and treating such cases. On the other hand, as clinicians, we should be able to recognize a rare condition as well. Hence, the vital question is: How do you differentiate if the question is about a “horse” or a “zebra?”
If the question is about a “zebra”, the question will have some information within the case vignette regarding the demography of the patient, which will be unusual for that particular disease, like the age of onset, patient’s gender, or race or ethnicity. For example, if on the surgery board a classic case of acute cholelithiasis in an adolescent with no history of pregnancy is presented, the age of the patient should alert you to causes that are common in children and adolescents, and not common in adults, such as formation of black pigment stones in the presence of hemolytic disease.
Or, an example from another specialty: if on the psychiatry board, a case of a 45-year-old man with no family history of dementia presents with dementia, Alzheimer’s disease is not the most likely etiology unless there is trisomy 21 of a mosaic form in the patient, which has fewer stigmata of Down’s syndrome. Or, if the case vignette describes a young woman from East-Asia who presents to the emergency room in an unconscious state, one should think about why her race and ethnicity are mentioned in the vignette. Suppose the family members of the woman tell you that the patient has immigrated from a rural area in a province of China and has been severely dejected about her move. Intentional poisoning with an organophosphate herbicide may be the cause of her condition, as discussed further below.
Or perhaps the tip-off in the question that it is a “zebra” is it has information about the clinical presentation which does not fit. An example is a patient that presents with abdominal distention and symptoms associated with acute intestinal obstruction who is also taking an antidepressant. The initial impression can lead to a diagnosis of intestinal obstruction, which might be true. However, the fact that the question is providing information about antidepressant use as well should be enough to trigger thoughts of other possible clinical hypotheses.
Another type of question that is a “zebra” might be one that provides a clue regarding the nature of the setting of the disease. The question can provide information of the recent history of traveling abroad (tropical country), unusual hobby or activity (mountain climbing), or information about the occupation of the patient, which is especially important if the patient is a factory worker or in agriculture in rural areas. For example, organophosphate exposure is relatively common in individuals working on a farm or in rural communities because of readily available herbicides, which are organophosphate compounds. Organophosphates inhibit cholinesterase and can lead to muscarinic and nicotinic over-activity, resulting in death due to diaphragmatic failure; it is the most common way to commit suicide in young women in rural China. The previously mentioned example of the East-Asian women fits this explanation, as she might have knowledge about the lethal nature of these commonly available chemicals due to her rural background. The question has also mentioned her depressed state, which was also an important clue.
The formula for the board exams is “the information in the vignette is never included by accident”. All the facts, or any piece of information, mentioned in the questions are designed to help you choose diagnostic or treatment distinctions.
Example Exam Question from General Surgery
A 48-year-old man presents to the clinic with a history of on and off fever, weakness, and intermittent bouts of colicky abdominal pain along with jaundice for the past one month. He has recently moved back to the U.S. after living in Thailand for the past 20 years. MR Cholangiogram shows severe dilatation of posterior inferior segmental branch of a superior hepatic lobe, and it is filled with multiple stones. What is the most likely etiology of this patient’s condition?
- Chronic biliary fluke infection
- HAV infection
- Chronic pancreatitis
Clonorchis sinensis is a biliary fluke found in rice-growing regions of East Asia. This can cause biliary strictures, pre-disposing them to the formation of bile stones (brown pigment) throughout intra- and extrahepatic ducts. This is known as hepatolithiasis due to chronic infection. Symptomatic patients can present with on and off fever, anemia, colicky pain, jaundice, and secondary bacterial infections. Chronic biliary fluke infection is the most likely etiology in this patient due to his history of residence in Thailand, along with symptomatology of gallstone disease. Leishmaniasis is incorrect, as the presentation of the patient does not fit that of the signs and symptoms seen in Leishmaniasis. HAV infection is incorrect because a patient with hepatitis A can present with fatigue, nausea, vomiting, fever, hepatomegaly, jaundice, dark urine, anorexia, and rash. Amebiasis is incorrect because it is caused by E.histolytica, which either presents as amebic colitis with diarrhea or amebic liver abscess. Chronic pancreatitis is incorrect, as the symptomatology of the above patient does not fit the clinical presentation of a patient with chronic pancreatitis.