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USMLE® Step 1 Question of the Day: Lipid Transport

Osmosis Team
Published on Sep 22, 2021. Updated on Sep 22, 2021.

Each week, Osmosis shares a USMLE® Step 1-style practice question to test your knowledge of medical topics. Today's case involves a 28-year-old woman with two episodes of pancreatitis over the past year. Can you figure out the cause?

A 28-year-old woman is being evaluated by her primary care physician for recurrent pancreatitis. The patient has had two episodes of pancreatitis over the past year. She consumes a balanced diet and does not drink alcohol. Family history is notable for osteoarthritis in her father. Her temperature is 37.5°C (99.5°F), blood pressure is 121/72 mmHg, and pulse is 64/min. Physical examination of the skin is notable for multiple small red-yellow papules on the buttocks and shoulders. Laboratory testing reveals a serum triglyceride level of 830 mg/dL. Which of the following best describes the function of the protein that is most likely defective in this patient’s condition?

A. Breakdown of triglycerides into fatty acids

B. Uptake of lipoprotein remnants

C. Binding of low-density lipoprotein to its receptors

D. Conversion of free cholesterol to cholesteryl esters

E. Secretion of chylomicrons into the lymphatics

Scroll down to find the answer!


The correct answer to today's USMLE® Step 1 Question is...

A. Breakdown of triglycerides into fatty acids

Before we get to the Main Explanation, let's look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

The incorrect answers to today's USMLE® Step 1 Question are...

B. Uptake of lipoprotein remnants

Incorrect: Apolipoprotein E (ApoE) is involved in the uptake of lipoprotein remnants. An ApoE defect presents with premature atherosclerosis, xanthomas, and increased risk of developing acute coronary syndrome. Although this patient has xanthomas, it would be atypical for an ApoE defect to cause recurrent pancreatitis.

C. Binding of low density lipoprotein to its receptors

Incorrect: Apolipoprotein B-100 mediates the binding of low density lipoproteins (LDLs) to their receptors. A defect leads to type II familial hypercholesterolemia, which presents with atherosclerosis, early onset of coronary artery disease, tendon xanthomas, and corneal arcus. In contrast, this patient has no signs of coronary artery disease or tendon xanthomas.

D. Conversion of free cholesterol to cholesteryl esters

Incorrect: A defect in lecithin-cholesterol acyltransferase (LCAT), or its cofactor apolipoprotein A-I, would impair this process. Defects in apolipoprotein A-I can lead to Tangier disease, which presents with cholesterol deposits in the liver, spleen, bone marrow, and lymphatics. Patients with Tangier disease may also present with mild triglyceridemia. In contrast, this patient has a markedly elevated triglyceride level, which favors another diagnosis.

E. Secretion of chylomicrons into the lymphatics

Incorrect: Apolipoprotein B-48 mediates this process. A defect in this protein is seen in abetalipoproteinemia. This condition presents in infancy with fat malabsorption and failure to thrive. Patients may also develop deficiencies in fat-soluble vitamins. The patient in this vignette has none of these symptoms.

Main Explanation

This patient has type I familial hyperlipidemia, which is most often due to a defect in lipoprotein lipase. Lipoprotein lipase is an enzyme that breaks down triglycerides into fatty acids and glycerol. The condition presents with pancreatitis and eruptive xanthomas.

After a meal, fats are taken up by intestinal cells, packaged into chylomicrons, and released into the bloodstream. Capillary lipoprotein lipase breaks down chylomicrons in order to free the triglycerides, which are absorbed by nearby tissues.

Chylomicron remnants, which contain cholesterol and small amounts of triglyceride, then travel to the liver to deposit the leftover lipids. The liver will combine these remnants with more, newly synthesized fatty acids and cholesterol, and it will package them together into very low-density lipoproteins (VLDLs).

VLDLs are released into the blood where lipoprotein lipase breaks them down again to release triglycerides for nearby tissues to use. As triglycerides leave the VLDL, it becomes an intermediate density lipoprotein (IDL) and eventually a low density lipoprotein (LDL). LDLs are ultimately endocytosed by liver and peripheral tissue cells expressing LDL receptors.

Without a functional lipoprotein lipase, the breakdown of chylomicrons and their triglyceride content is prevented, resulting in massive triglyceride accumulation within the blood with subsequent clinical consequences (e.g., recurrent pancreatitis, eruptive xanthomas).

Major Takeaway

Lipoprotein lipase helps breakdown triglycerides into fatty acids and glycerol so they can be absorbed by tissue. A defect in this enzyme leads to type I familial hyperlipidemia, which causes pancreatitis and eruptive xanthomas.

References

Falko, J.M. (2018) Familial chylomicronemia syndrome: A clinical guide of endocrinologists. Endocrine Practice. 24(8), 756-763. Doi: 10.4158/EP-2018-0157.

Regmi, M., Rehman, A. (2019) Familial hyperlipidemia type 1. StatPearls [Internet]. Web Address: https://www.ncbi.nlm.nih.gov/books/NBK551655/.

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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB.