THE USMLE TIMES - #105
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Welcome to issue #105 of The USMLE Times! Welcome to all new members—and welcome back to those returning!
I’m Paul Ciurysek, MD, founder of The USMLE Guys, and this is The USMLE Times. This daily newsletter aims to provide super high-yield USMLE concepts commonly tested on exam day. All content is FREE! If you’d like to work with me and my team, please see the options at the bottom of today’s newsletter. If you’d like to support my efforts, please share the newsletter with a friend.
Here’s what’s on tap for issue #105 of the USMLE Times:
Question of the Week (Genetic Disorders)
This week’s video training (Biochemistry Drill Session)
Question deep-dive & breakdown
Let’s dive in!
A 7-year-old boy is brought to the clinic with a persistent, productive cough, recurrent respiratory infections, and frequent episodes of diarrhea. His mother reports that he has difficulty gaining weight despite a good appetite and produces large, greasy stools. Family history reveals a maternal uncle with similar symptoms. Physical examination shows digital clubbing and bilateral coarse lung crackles. A sweat chloride test is positive. Which of the following is most commonly associated with this patient’s underlying condition?
A) Pancreatic dysfunction leading to impaired enzyme secretion and poor digestion
B) Decreased bile flow resulting in fat malabsorption and steatorrhea
C) Hepatic enzyme deficiency causing difficulty in fat digestion and nutrient absorption
D) Dysfunctional intestinal absorption leading to fat-soluble vitamin deficiencies
E) Gallbladder dysfunction with reduced bile acid production and poor fat emulsification
The answer & question breakdown is at the bottom of the post 👇🏼
🔗 LINKS TO RECENT POSTS
USMLE Times Issue #104 - READ HERE
USMLE Practice Question #106 - TRY IT HERE
USMLE Practice Question #105 - TRY IT HERE
This week’s recommended video 👇🏼
ANSWER + QUESTION BREAKDOWN
The mental model used to answer this question comes from our detailed test-taking skills masterclass (check it out if you want to elevate your USMLE test-taking skills) Here’s how to think through this question:
Step 1. Read the vignette and get the actual question: “Which of the following findings is most commonly associated with this patient’s underlying condition?”
Step 2. Determine if this is a 1st order, 2nd order, or 3rd order question. This is 2nd order: 1st. Make a diagnosis; 2nd: Find the answer choice associated with the underlying condition.
Step 3. Based on your thoughts from step 2 above, ask yourself: “What’s likely to be present due to the patient’s condition?”
Step 4. Look at the answer choices and select the one that matches your thoughts from step 3 above.
** Aim to take this mental model into all of your USMLE practice questions **
GENERAL ANALYSIS
This vignette describes a 7-year-old boy with a history of persistent productive cough, recurrent respiratory infections, frequent diarrhea, and difficulty gaining weight despite a good appetite. His mother reports large, greasy stools and a physical exam reveals digital clubbing and bilateral coarse lung crackles. A positive sweat chloride test confirms the diagnosis of cystic fibrosis.
Key Concepts:
Cystic fibrosis (CF) is an autosomal recessive disorder caused by mutations in the CFTR gene. These mutations lead to defective chloride and bicarbonate transport across epithelial cells, resulting in thickened secretions that affect multiple organs, including the lungs, pancreas, liver, and intestines.
In the digestive system, CF primarily affects the pancreas by causing thick secretions that block pancreatic ducts, leading to pancreatic insufficiency. This results in malabsorption of fats and fat-soluble vitamins (A, D, E, K), leading to steatorrhea (greasy stools) and poor weight gain despite adequate caloric intake.
ANSWER CHOICES:
CHOICE A: Pancreatic dysfunction leading to impaired enzyme secretion and poor digestion
Explanation: In cystic fibrosis, thickened secretions block the pancreatic ducts, preventing the release of digestive enzymes (lipase, amylase, protease) into the small intestine. This leads to malabsorption of fats and proteins, resulting in steatorrhea (greasy stools), poor weight gain, and fat-soluble vitamin deficiencies.
Why Correct: This is the correct answer. The patient's symptoms—greasy stools, poor weight gain despite a good appetite—are classic signs of pancreatic insufficiency, which is a hallmark of cystic fibrosis. Pancreatic dysfunction is one of the earliest manifestations of CF and leads to impaired digestion due to insufficient enzyme secretion.
CHOICE B: Decreased bile flow resulting in fat malabsorption and steatorrhea
Explanation: Decreased bile flow (cholestasis) can lead to fat malabsorption because bile is necessary for emulsifying fats in the intestine. However, this is not the primary mechanism in cystic fibrosis.
Why Incorrect: While CF can affect the liver and biliary system in some patients, leading to cholestasis or liver disease later in life, this patient’s symptoms are more consistent with pancreatic insufficiency rather than bile flow issues.
CHOICE C: Hepatic enzyme deficiency causing difficulty in fat digestion and nutrient absorption
Explanation: Hepatic enzyme deficiencies are not typically associated with cystic fibrosis. The liver produces bile acids that aid in fat digestion but do not produce digestive enzymes.
Why Incorrect: The primary issue in CF-related malabsorption is pancreatic enzyme deficiency due to ductal obstruction from thick secretions. Hepatic enzyme deficiency is not involved in this process.
CHOICE D: Dysfunctional intestinal absorption leading to fat-soluble vitamin deficiencies
Explanation: While CF patients may develop fat-soluble vitamin deficiencies (A, D, E, K), these deficiencies are secondary to impaired digestion due to pancreatic insufficiency rather than a primary intestinal absorption issue.
Why Incorrect: The root cause of malabsorption in CF is pancreatic dysfunction rather than intrinsic intestinal absorption defects.
CHOICE E: Gallbladder dysfunction with reduced bile acid production and poor fat emulsification
Explanation: Gallbladder dysfunction can lead to reduced bile acid secretion and impaired fat digestion; however, this is not the primary mechanism in cystic fibrosis.
Why Incorrect: Gallbladder dysfunction is not a primary feature of cystic fibrosis. The patient’s symptoms are better explained by pancreatic insufficiency.
THE VERDICT…
The most likely cause of this patient’s symptoms—recurrent respiratory infections, productive cough, greasy stools (steatorrhea), difficulty gaining weight despite a good appetite—is cystic fibrosis, confirmed by a positive sweat chloride test.
In CF patients, thickened secretions block the pancreatic ducts early in life, leading to impaired secretion of digestive enzymes such as lipase and amylase. This results in poor digestion of fats and proteins, causing steatorrhea and malnutrition despite adequate caloric intake.
Therefore, the most common finding associated with cystic fibrosis is pancreatic dysfunction, which leads to impaired enzyme secretion and poor digestion.
Correct answer: A) Pancreatic dysfunction leading to impaired enzyme secretion and poor digestion
That’s it for issue #105 of The USMLE Times!
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See ya next week 👋