A scientist is trying to design a drug to modulate cellular metabolism in the treatment of obesity. Specifically, he is interested in understanding how fats are processed in adipocytes in response to different energy states. His target is a protein within these cells that catalyzes catabolism of an energy source. The products of this reaction are subsequently used in gluconeogenesis or β-oxidation. Which of the following is true of the most likely protein that is being studied by this scientist?

It is stimulated by epinephrine

It is inhibited by acetylcholine

A 12-year-old boy is brought to the emergency department because of acute onset abdominal pain. On arrival, he also complains of nausea and shortness of breath in addition to epigastric pain. He has previously been admitted to the hospital several times for respiratory infections with Pseudomonas species and uses a nebulizer and a chest wall oscillation vest at home. The patient's acute condition is found to be due to premature activation of an enzyme that normally interacts with the brush border. Which of the following describes the activity of this enzyme?

Activates pancreatic enzyme precursors

A 21-year-old college student comes to the emergency department because of a two-day history of vomiting and epigastric pain that radiates to the back. He has a history of atopic dermatitis and Hashimoto thyroiditis. His only medication is levothyroxine. He has not received any routine vaccinations. He drinks 1–2 beers on the weekends and occasionally smokes marijuana. The patient appears distressed and is diaphoretic. His temperature is 37.9°C (100.3°F), pulse is 105/min, respirations are 16/min, and blood pressure is 130/78 mm Hg. Physical examination shows abdominal distention with tenderness to palpation in the epigastrium. There is no guarding or rebound tenderness. Skin examination shows several clusters of yellow plaques over the trunk and extensor surfaces of the extremities. Hemoglobin concentration is 15.2 g/dL and serum calcium concentration is 7.9 mg/dL. Which of the following is the most appropriate next step in evaluation?

Obtain an upright x-ray of the abdomen

Perform a pilocarpine-induced sweat test

A 39-year-old woman presents to her primary care physician because she has been experiencing intermittent abdominal pain for the last 2 weeks. She says that the pain is squeezing in nature, is located in the right upper quadrant, and is particularly severe after eating a meal. After a diagnosis is made, the patient asks why the pain gets worse after eating. The physician explains that food is detected by the gastrointestinal tract through numerous receptors and that this information is transmitted to other parts of the body to cause compensatory changes. The neurons responsible for transmitting this information are most likely located in a layer of the intestine that has which of the following characteristics?

Contracts to generate peristaltic waves

Contracts to generate local movement in mucosa

Contains cells that primarily absorb nutrients

Connective tissue that envelops the other layers

Contains large blood vessels and large lymphatic vessels

Lipid digestion and absorption for USMLE Step 3: High-Yield Physiology Lessons

Lipid Digestion Kickoff - The First Cut

Lipid Digestion and Absorption in the GI Tract

Dietary Lipids: Primarily Triglycerides (TGs), with smaller amounts of cholesterol and phospholipids.
Mouth: Digestion starts with lingual lipase, secreted by glands on the tongue.
Stomach: Gastric lipase, secreted by chief cells, continues the process started in the mouth.
- These acid-stable lipases hydrolyze ~10-30% of dietary TGs into fatty acids and diglycerides.

⭐ Lingual and gastric lipases are crucial in infants (for milk fat) and in patients with pancreatic insufficiency, as they can function without bile salts.

Small Intestine Action - Bile & Pancreas Party

Trigger: Acidic, fatty chyme entering the duodenum stimulates enteroendocrine cells to release key hormones.
- Cholecystokinin (CCK): Released in response to fats and proteins.
- Secretin: Released in response to acid.
Hormonal Actions:
- CCK triggers gallbladder contraction to release bile and stimulates the pancreas to secrete pancreatic lipase and colipase.
- Secretin causes pancreatic duct cells to release bicarbonate ($HCO_3^-$) to neutralize stomach acid.
Digestion Cascade:
- Emulsification: Bile salts break large fat globules into smaller droplets, vastly increasing the surface area for enzymatic action.
- Hydrolysis: Pancreatic lipase, activated by colipase, breaks down triglycerides into free fatty acids and monoglycerides.
- Micelle Formation: Bile salts then enclose these lipid products, forming water-soluble micelles that transport lipids to the intestinal lining for absorption.

⭐ Colipase, also secreted by the pancreas, is essential for relieving the inhibition of pancreatic lipase caused by bile salts coating the emulsion droplets.

📌 Cholecystokinin (CCK) Causes Contraction of the gallbladder.

Lipid Digestion and Absorption Pathway

Absorption & Packaging - The Great Escape

Lipid digestion and absorption in intestinal cell

Lipids travel from the intestinal lumen into the lymphatic system in a multi-step process. Micelles deliver fatty acids and monoglycerides to the enterocyte brush border for diffusion. Inside the cell, they are re-formed into triglycerides.

These triglycerides are packaged with Apolipoprotein B-48 to create chylomicrons, which are then secreted from the enterocyte's basolateral membrane into lacteals (lymphatics), bypassing the portal circulation initially.

⭐ Short- and medium-chain fatty acids (<12 carbons) are more water-soluble and can be absorbed directly into the portal blood without requiring micelle formation or packaging into chylomicrons.

📌 ApoB-48: The "Boarding pass" for chylomicrons to exit the enterocyte.

Clinical Correlations - When Fats Fight Back

Condition/Drug	Mechanism of Steatorrhea	Clinical Context & Features
Pancreatic Insufficiency	Deficient pancreatic lipase and colipase secretion prevents fat breakdown.	Seen in Cystic Fibrosis, chronic pancreatitis. Results in ADEK vitamin deficiencies.
Bile Salt Deficiency	Impaired micelle formation due to a reduced bile salt pool.	Caused by ileal resection or cholestasis. Leads to poor absorption of fats & vitamins.
Abetalipoproteinemia	Inability to form chylomicrons (requires ApoB-48) & VLDL (ApoB-100).	Rare autosomal recessive disorder. Presents with acanthocytic RBCs, neuro deficits.
Orlistat (Drug)	Covalently inhibits both gastric and pancreatic lipases, blocking digestion.	A common weight loss medication. 📌 Orlistat makes fat stationary in the gut.
Olestra (Additive)	A chemically engineered, indigestible fat substitute that is not absorbed.	Found in some "fat-free" snacks. Can cause abdominal cramping and loose stools.

Lingual and gastric lipases initiate fat digestion, but the majority occurs in the small intestine.

Bile salts emulsify fats, while pancreatic lipase and colipase perform the bulk of chemical digestion.

Digested lipids form micelles to cross the enterocyte membrane.

Inside the cell, fatty acids are re-esterified and packaged into chylomicrons.

Chylomicrons require ApoB-48 for assembly and are secreted into lacteals, entering the lymphatic system.

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