GI physiology (digestion, absorption, motility) Practice Questions

Q: A 50-year-old woman with long-standing diabetes presents with severe, watery diarrhea that wakes her at night. Stool studies show normal osmotic gap and negative stool cultures. Colonoscopy is normal. Trial of fasting does not improve diarrhea. Gastric emptying study shows delayed emptying. What neurotransmitter deficiency in the enteric nervous system best explains both her gastric and colonic dysmotility?

Nitric oxide. ***Nitric oxide*** - **Nitric oxide (NO)** is the primary **inhibitory neurotransmitter** in the enteric nervous system responsible for mediating **receptive relaxation** of the stomach and smooth muscle inhibition during peristalsis. - In **diabetic autonomic neuropathy**, loss of nitrergic neurons leads to **gastroparesis** (failed pyloric relaxation) and **diabetic diarrhea** due to uncoordinated colonic motility and loss of descending inhibition. *Acetylcholine* - **Acetylcholine** is the primary **excitatory neurotransmitter** responsible for muscle contraction; a deficiency would likely lead to paralytic ileus rather than secretory-pattern diarrhea. - Though diabetic neuropathy affects cholinergic fibers, the hallmark of the inhibitory dysmotility seen in **gastroparesis** is specifically linked to the NO pathway. *Serotonin* - **Serotonin (5-HT)** is predominantly involved in the initiation of the **peristaltic reflex** and intestinal secretion via enterochromaffin cells. - While 5-HT levels can be altered in IBS, it is not the classic neurotransmitter deficiency cited for the specific combination of **delayed gastric emptying** and autonomic diarrhea in diabetics. *Substance P* - **Substance P** acts as a **co-transmitter** with acetylcholine to promote smooth muscle contraction and pain transmission. - Deficiency of Substance P would impair motor activity but does not explain the loss of **inhibitory control** and relaxation necessary for normal gastric emptying. *Vasoactive intestinal peptide* - **Vasoactive intestinal peptide (VIP)** is an inhibitory neurotransmitter that promotes intestinal secretion and smooth muscle relaxation. - While VIP is involved in relaxation, **Nitric oxide** is considered the more critical mediator for the **pyloric and fundal relaxation** that is specifically impaired in diabetic gastropathy.

Q: A 68-year-old man with cirrhosis develops hepatic encephalopathy. He is started on lactulose and rifaximin. His mental status improves within 48 hours. Beyond decreasing ammonia-producing bacteria, lactulose exerts additional beneficial effects through altering colonic pH. Analyze how pH modification affects ammonia metabolism in the colon.

Acidic pH converts NH3 to NH4+ which is poorly absorbed. ***Acidic pH converts NH3 to NH4+ which is poorly absorbed*** - Lactulose is a non-absorbable disaccharide metabolized by colonic bacteria into **lactic and acetic acids**, which significantly lowers the **luminal pH**. - This acidic environment promotes the protonation of diffusible **ammonia (NH3)** into non-absorbable **ammonium (NH4+)**, a process known as **ion trapping**, which prevents its entry into the systemic circulation. *Decreased pH enhances ammonia oxidation by colonocytes* - Colonocytes do not primarily use oxidation as a mechanism for clearing ammonia; instead, they are often a source of ammonia via **glutamine metabolism**. - The primary benefit of low pH in **hepatic encephalopathy** is the physical trapping of ions in the lumen, not an alteration in cellular oxidative pathways. *Low pH inhibits bacterial urease activity* - While extreme pH changes can affect enzymes, the primary therapeutic mechanism of lactulose is the **acidification of the gut** and the resultant **protonation of ammonia**. - Rifaximin, not lactulose, is the agent primarily responsible for targeting and inhibiting the **ammonia-producing bacteria** themselves. *Low pH stimulates glutamine synthetase in colonic mucosa* - **Glutamine synthetase** is mainly active in the **liver, brain, and muscle** to neutralize ammonia; it is not the key mechanism of action for lactulose in the colon. - Low colonic pH focuses on the **extracellular conversion** of ammonia to ammonium rather than intracellular enzymatic synthesis. *Acidification promotes conversion of ammonia to urea* - The **urea cycle** occurs exclusively in the **liver** (hepatocytes) and is not stimulated by the acidification of the distal colon. - In patients with **cirrhosis**, the liver's ability to convert ammonia to urea is already severely compromised, which is why alternative clearance methods are required.

Q: A 30-year-old woman presents with chronic diarrhea and a 15-pound weight loss. Stool studies show increased osmotic gap and pH of 5.0. Hydrogen breath test shows elevated hydrogen levels 30 minutes after lactose ingestion. Duodenal biopsy shows normal villous architecture with preserved brush border. What is the most likely enzyme deficiency?

Lactase. ***Lactase*** - The positive **hydrogen breath test** following lactose ingestion is diagnostic of **lactase deficiency**, as undigested lactose is fermented by colonic bacteria to produce hydrogen gas. - The **increased osmotic gap** and **low stool pH (5.0)** reflect the presence of unabsorbed sugars and the production of **short-chain fatty acids** by intestinal flora. *Maltase-glucoamylase* - This enzyme deficiency is extremely rare and would primarily result in symptoms after consuming **starch or maltose**, rather than lactose. - A hydrogen breath test specifically using lactose would not be positive in isolated maltase deficiency. *Sucrase-isomaltase* - Deficiency in this enzyme causes intolerance to **sucrose (table sugar)** and starch, leading to symptoms early in childhood. - While it presents with similar osmotic diarrhea, it is not triggered by **lactose ingestion** or diagnosed via a lactose-specific breath test. *Trehalase* - Trehalase is an enzyme specifically required to digest **trehalose**, a sugar found in **mushrooms**. - Deficiency would cause symptoms only after mushroom consumption and would not explain a positive **lactose breath test**. *Pancreatic amylase* - Pancreatic amylase deficiency results in **maldigestion of complex carbohydrates** (starches) and is usually associated with **pancreatic insufficiency** and steatorrhea. - Unlike disaccharidase deficiencies, it would typically be accompanied by signs of **protein or fat malabsorption** and would not result in a normal duodenal biopsy appearance of the brush border.

Q: A 42-year-old woman undergoes cholecystectomy. During surgery, the common bile duct is inadvertently ligated. Over the next week, she develops jaundice and pale stools. Laboratory studies show elevated direct bilirubin and alkaline phosphatase, but normal lipase. Despite complete biliary obstruction, she does not develop severe fat malabsorption. What physiologic mechanism best explains preserved fat digestion?

Gastric lipase can digest up to 30% of dietary fat. ***Gastric lipase can digest up to 30% of dietary fat*** - **Gastric lipase** (secreted by chief cells) and **lingual lipase** are acid-stable enzymes that can hydrolyze dietary triglycerides into fatty acids and diglycerides without needing **bile salts** or **colipase**. - This secondary mechanism ensures that up to **10-30% of dietary fat** is digested even in cases of complete **biliary obstruction** or pancreatic insufficiency, preventing total malabsorption. *Intestinal mucosal lipase compensates for bile salt deficiency* - There is no significant "intestinal mucosal lipase" that acts on bulk dietary triglycerides to compensate for a lack of **bile-mediated emulsification**. - While enterocytes possess **intracellular lipases** for processing absorbed lipids, they do not facilitate significant **luminal digestion** of fat. *Pancreatic lipase functions adequately without bile salts* - **Pancreatic lipase** requires **bile salts** to create a large surface area via **emulsification** and for the formation of **micelles** to carry lipids to the brush border. - Without **colipase** and bile salts, pancreatic lipase is easily displaced from the fat droplet, leading to a significant decrease in its functional efficiency. *Salivary lipase becomes upregulated in biliary obstruction* - **Lingual lipase** does contribute to lipid digestion in the stomach, but there is no physiological evidence that it becomes significantly **upregulated** to compensate for biliary issues. - While it initiates digestion, its total capacity is limited compared to the primary role of the **pancreatic-biliary** axis. *Colonic bacteria produce enzymes that digest fat* - **Colonic bacteria** primarily digest undigested carbohydrates and proteins through **fermentation**, not lipids. - Excessive undigested fat reaching the colon usually results in **steatorrhea** and osmotic diarrhea rather than being utilized or digested by microflora.

Question 1

A 50-year-old woman with long-standing diabetes presents with severe, watery diarrhea that wakes her at night. Stool studies show normal osmotic gap and negative stool cultures. Colonoscopy is normal. Trial of fasting does not improve diarrhea. Gastric emptying study shows delayed emptying. What neurotransmitter deficiency in the enteric nervous system best explains both her gastric and colonic dysmotility?

Accepted Answer

Nitric oxide

Answer

Acetylcholine

Answer

Serotonin

Answer

Substance P

Answer

Vasoactive intestinal peptide

Question 2

A 25-year-old man undergoes extensive small bowel resection following trauma, leaving 40 cm of jejunum anastomosed to the ascending colon. The ileocecal valve is removed. Six months postoperatively, he requires total parenteral nutrition. Despite this, he develops calcium oxalate kidney stones. Evaluate the mechanism linking his intestinal resection to nephrolithiasis.

Accepted Answer

Increased enteric oxalate absorption from calcium binding to malabsorbed fatty acids

Answer

Decreased urinary citrate from chronic metabolic acidosis

Answer

Hyperparathyroidism from vitamin D malabsorption causing hypercalciuria

Answer

Increased colonic absorption of oxalate due to enhanced permeability from bile salts

Answer

Dehydration from chronic diarrhea concentrating urinary oxalate

Question 3

A 68-year-old man with cirrhosis develops hepatic encephalopathy. He is started on lactulose and rifaximin. His mental status improves within 48 hours. Beyond decreasing ammonia-producing bacteria, lactulose exerts additional beneficial effects through altering colonic pH. Analyze how pH modification affects ammonia metabolism in the colon.

Accepted Answer

Acidic pH converts NH3 to NH4+ which is poorly absorbed

Answer

Decreased pH enhances ammonia oxidation by colonocytes

Answer

Low pH inhibits bacterial urease activity

Answer

Low pH stimulates glutamine synthetase in colonic mucosa

Answer

Acidification promotes conversion of ammonia to urea

Question 4

A 30-year-old woman presents with chronic diarrhea and a 15-pound weight loss. Stool studies show increased osmotic gap and pH of 5.0. Hydrogen breath test shows elevated hydrogen levels 30 minutes after lactose ingestion. Duodenal biopsy shows normal villous architecture with preserved brush border. What is the most likely enzyme deficiency?

Accepted Answer

Lactase

Answer

Maltase-glucoamylase

Answer

Sucrase-isomaltase

Answer

Trehalase

Answer

Pancreatic amylase

Question 5

A 42-year-old woman undergoes cholecystectomy. During surgery, the common bile duct is inadvertently ligated. Over the next week, she develops jaundice and pale stools. Laboratory studies show elevated direct bilirubin and alkaline phosphatase, but normal lipase. Despite complete biliary obstruction, she does not develop severe fat malabsorption. What physiologic mechanism best explains preserved fat digestion?

Accepted Answer

Gastric lipase can digest up to 30% of dietary fat

Answer

Intestinal mucosal lipase compensates for bile salt deficiency

Answer

Pancreatic lipase functions adequately without bile salts

Answer

Salivary lipase becomes upregulated in biliary obstruction

Answer

Colonic bacteria produce enzymes that digest fat

GI physiology (digestion, absorption, motility) — MCQs

GI physiology (digestion, absorption, motility) — MCQs

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