The board exams pride themselves on not presenting questions that may trick you. In other words, the questions are fair and clinically relevant. This means that the majority of the questions will be related to a common form of a medical disorder. After all, these are the conditions that most of the physicians spend their days assessing and treating. However, it is also important for the clinician to identify the rare disorder. Hence, the question is, how to differentiate whether the question is regarding a “horse” or a “zebra?”
A question that is a “zebra” will contain some information about the demographics of the patient that is not usual for that disease, such as it being an uncommon occurrence in that patient’s racial or ethnic group, in their gender, or at their age. For instance, if the question describes a patient under the age of 20 years with high blood pressure that is difficult to control, then you should be alerted by the age of the patient to think about causes of hypertension that are not common in older adults (e.g., pheochromocytoma).
Or perhaps the question may suggest a “zebra” if it contains some information regarding the clinical presentation that seems totally out of place. An example of this would be the case of a known asthma patient who complains of symptoms of cough, shortness of breath, wheezing, and chest tightness, particularly when he is sleeping at night. The vignette describes that the patient is using “some asthma inhaler” for his condition. The first impression that you may get from this information is that the current asthma treatment is probably ineffective, which may very well be the case. However, an alternative explanation would be that the inhaler that the patient is using is a short-acting bronchodilator, which is insufficient to cover the hours of sleep at night, thus resulting in nocturnal symptoms as the plasma levels of the medication fall. The occurrence of nocturnal asthma attacks and the use of “some asthma inhaler” should be enough to at least make you think beyond your first clinical hypothesis.
Another type of clue for a “zebra” would be related to the setting or nature of the disease. Perhaps the question may mention that the patient has a recent history of travel to a tropical country or being involved in an unusual activity, such as spelunking or mountain climbing, or the occupational history of the patient may be described. In the latter case, close attention needs to be given to the information indicating that the patient works in an industrial or agricultural setting. For example, if the vignette describes a patient who presents for the evaluation of chronic nonproductive cough accompanied by shortness of breath, and has a longstanding history of working as a plumber, close attention should be given to the symptoms as well as the patient’s occupation. This information may guide you in the direction of asbestos-associated lung disease, which is an occupational lung disease.
For the purposes of board exams, it is important to remember that nothing has been included in the clinical vignette by accident. Every piece of information placed in the question is there to lead you towards one treatment path or another, or towards one diagnostic path or another. All the information is there to assist you in making diagnostic and/or treatment distinctions.
Example Exam Question from Gastroenterology:
Question: A 49-year-old woman has been referred to the clinic for the evaluation of chronic greasy, oily, fatty, and non-bloody diarrhea accompanied by bloating, fatigue, and lethargy for the past one year. History of weight loss during this time period is also present. Antiendomysial, antitransglutaminase, and antigliadin antibodies are negative. Duodenal biopsy reveals subtotal villous blunting. The patient has been living in Indonesia because of her job for the last 18 months and has recently returned to the U.S. No significant past medical or surgical history is present. Which of the following is the most likely diagnosis?
- Celiac disease
- Inflammatory bowel disease (IBD)
- Tropical sprue
- Chronic pancreatitis
- Irritable bowel syndrome (IBS)
Tropical sprue is the most likely diagnosis in this patient. Tropical sprue is a chronic, progressive malabsorption disorder that is associated with abnormalities of the structure and function of the small intestine. It may present with chronic diarrhea, anorexia, abdominal distension, fatigue, or weight loss, and is one of the causes of subtotal villous blunting. The disease occurs mainly in tropical areas such as West Indies, Southern India, Indonesia, and Malaysia.
Celiac disease is an important differential diagnosis, as it may also present with similar symptoms and may also cause villous blunting. However, in celiac disease, antiendomysial, antitransglutaminase, and antigliadin antibodies are usually present. Moreover, the history of the patient living in the tropics for a period of 18 months points you more towards tropical sprue.
Inflammatory bowel disease (IBD) may present with fever, diarrhea, weight loss, occasionally rectal bleeding, and abdominal pain. Extra-intestinal clinical manifestations may also be present. Patients with chronic pancreatitis usually have a history of repeated episodes of acute pancreatitis from gallstones or alcohol intake. Irritable bowel syndrome (IBS) is usually characterized by recurrent abdominal pain and changes in pattern of the bowel movements in the absence of a structural abnormality of the gut. None of these three conditions is usually associated with subtotal villous blunting.
El-Omar E, McLean MH. Gastroenterology. In: Ralston SH, Penman ID, Strachan MWJ, Hobson RP. Davidson’s Principles and Practice of Medicine. 23rd ed. Edinburgh: Churchill Livingstone/Elsevier; 2018. pp. 763-844. Ch.21.
Langenberg M, Wismans P, van Genderen P. Distinguishing tropical sprue from celiac disease in returning travellers with chronic diarrhoea: a diagnostic challenge? Travel Med Infect Dis. 2014;12(4):401–5. https://doi.org/10.1016/j.tmaid.2014.05.001.