Gastrointestinal System — MCQs
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What is the main enzyme involved in the digestion of dietary fats?
Pepsinogen is activated by?
Maximum water reabsorption in the gastrointestinal tract occurs in:
Which of the following is passively absorbed in gut?
What is the reflex in which there is inhibition of gastric emptying when there is acid and hypertonic solution in the duodenum?
Small intestinal peristalsis is controlled by :
Lower esophageal sphincter pressure is increased by all of the following substances, EXCEPT:
Cholecystokinin is produced from:
Secretion of bile out of hepatocytes occurs via?
What is the primary hormonal action of cholecystokinin (CCK) in response to fat and protein in the duodenum?
Gastrointestinal System Indian Medical PG Practice Questions and MCQs
Question 521: What is the main enzyme involved in the digestion of dietary fats?
- A. Lingual lipase
- B. Gastric lipase
- C. Pancreatic lipase (Correct Answer)
- D. Phospholipase
Explanation: ***Pancreatic lipase*** - **Pancreatic lipase** is the primary enzyme responsible for the digestion of dietary **triglycerides** in the small intestine. - It hydrolyzes triglycerides into **monoglycerides** and **fatty acids**, which can then be absorbed by the intestinal lining. *Lingual lipase* - **Lingual lipase** is secreted in the mouth and begins the digestion of some dietary fats, particularly **short-chain** and **medium-chain triglycerides**. - Its activity is limited and contributes only a small percentage to overall fat digestion, primarily in the stomach due to its acid stability. *Gastric lipase* - **Gastric lipase** is produced in the stomach and primarily digests **short-chain** and **medium-chain triglycerides**, especially important for infants. - While it initiates some fat digestion, its contribution to the overall breakdown of dietary fats is minor compared to pancreatic lipase. *Phospholipase* - **Phospholipase** is an enzyme that specifically breaks down **phospholipids** (not triglycerides) into fatty acids and other lipophilic substances. - It plays a role in the digestion of membrane lipids, but not the bulk of dietary triglycerides.
Question 522: Pepsinogen is activated by?
- A. H+ (Correct Answer)
- B. Gastrin
- C. Enterokinase
- D. Trypsin
Explanation: ***H+*** - **Pepsinogen**, a zymogen, is activated by the **acidic environment** created by the secretion of **hydrochloric acid (H+)** in the stomach. - The H+ ions cause a conformational change in pepsinogen, leading to the autocatalytic cleavage of a small peptide segment, forming the active enzyme **pepsin**. *Enterokinase* - **Enterokinase** is an enzyme found in the brush border of the small intestine that activates **trypsinogen to trypsin**. - It plays a crucial role in the activation of pancreatic proteases, not gastric pepsinogen. *Gastrin* - **Gastrin** is a hormone that stimulates the secretion of **hydrochloric acid (HCl)** and pepsinogen by the stomach's parietal and chief cells, respectively. - While it *promotes* the conditions for pepsinogen activation, it does not directly activate pepsinogen itself. *Trypsin* - **Trypsin** is a protease found in the small intestine, formed from **trypsinogen** through the action of enterokinase. - Its primary role is to digest proteins and activate other pancreatic zymogens, not gastric pepsinogen.
Question 523: Maximum water reabsorption in the gastrointestinal tract occurs in:
- A. Colon
- B. Jejunum (Correct Answer)
- C. Stomach
- D. Ileum
Explanation: ***Jejunum*** - The **jejunum** is the site of **maximum water absorption** in the gastrointestinal tract by volume. - Approximately **5-6 liters of water** are absorbed daily in the jejunum, which is the largest absolute amount of any GI segment. - While the jejunum is known primarily for nutrient absorption (carbohydrates, proteins, fats), it handles the bulk of fluid absorption due to the large volume of secretions (saliva, gastric juice, bile, pancreatic juice) entering the upper GI tract. *Colon* - The **colon** absorbs approximately **1-2 liters of water** daily, which is significantly less than the jejunum. - However, the colon is crucial for **concentrating feces** and has high efficiency in water reabsorption (absorbs ~90% of water entering it). - It plays a vital role in final water conservation, but not maximum absolute volume absorption. *Ileum* - The **ileum** absorbs approximately **2-3 liters of water** daily, along with bile acids and vitamin B12. - Its water absorption capacity is important but less than the jejunum. *Stomach* - The **stomach** has minimal water absorption capacity. - Only small amounts of water and certain substances like alcohol are absorbed here. - The stomach primarily functions in digestion and mixing food with gastric secretions.
Question 524: Which of the following is passively absorbed in gut?
- A. Glucose
- B. Lipids (Correct Answer)
- C. Fructose
- D. Amino acids
Explanation: ***Lipids*** - **Short-chain fatty acids** and **diglycerides** can be directly absorbed into epithelial cells via **passive diffusion** due to their lipid solubility. - **Micelles**, formed from longer-chain fatty acids and monoglycerides, diffuse across the unstirred water layer and release their contents, allowing these products to passively diffuse across the cell membrane. *Glucose* - **Glucose absorption** in the gut primarily occurs via **active transport** mechanisms, specifically the **SGLT1 transporter** (Na+-glucose cotransporter) and **GLUT2 transporter**. - While GLUT2 can facilitate some passive diffusion at very high concentrations, it is mainly involved in facilitated diffusion and overall glucose absorption is energy-dependent. *Fructose* - **Fructose absorption** primarily occurs through **facilitated diffusion** via the **GLUT5 transporter** in the small intestine. - This process is still a form of passive transport, but it requires a carrier protein and is not simple diffusion, making it distinct from the direct passive absorption of lipids. *Amino-acids* - **Amino acid absorption** predominantly involves **active transport systems** that are specific for different groups of amino acids (e.g., neutral, basic, acidic). - These transporters require energy (often co-transport with sodium) to move amino acids against their concentration gradient into intestinal cells.
Question 525: What is the reflex in which there is inhibition of gastric emptying when there is acid and hypertonic solution in the duodenum?
- A. Enterogastric (Correct Answer)
- B. Gastroileal
- C. Gastrocolic
- D. Myenteric
Explanation: ***Enterogastric*** - The **enterogastric reflex** is initiated when the duodenum detects the presence of acid and hypertonic solutions, signifying that the chyme is not yet ready for further digestion and absorption. - This reflex inhibits **gastric emptying** to allow more time for the stomach to process its contents and for the duodenum to neutralize the acid and dilute the hypertonic solution. *Gastroileal* - The **gastroileal reflex** increases motility in the ileum when the stomach is distended, facilitating the movement of chyme into the large intestine. - This reflex does not primarily involve the inhibition of gastric emptying due to duodenal contents. *Gastrocolic* - The **gastrocolic reflex** increases the motility of the colon in response to the stretching of the stomach by food. - Its main function is to prepare the large intestine for upcoming chyme and does not directly inhibit gastric emptying. *Myenteric* - The **myenteric plexus** (Auerbach's plexus) is a network of neurons located between the longitudinal and circular layers of the muscularis propria throughout the gastrointestinal tract. - While it plays a crucial role in controlling gut motility and is involved in numerous reflexes, it refers to a neural plexus rather than a specific reflex mechanism for inhibiting gastric emptying due to duodenal stimuli.
Question 526: Small intestinal peristalsis is controlled by :
- A. Meissner's plexus
- B. Vagus nerve
- C. Parasympathetic system
- D. Myenteric plexus (Correct Answer)
Explanation: ***Myenteric plexus*** - The **myenteric (Auerbach's) plexus** is located between the longitudinal and circular muscle layers of the muscularis propria and is primarily responsible for **controlling gastrointestinal motility**, including peristalsis. - Its neurons coordinate the contractions and relaxations of these muscle layers to propel contents through the alimentary canal. *Meissners plexus* - The **Meissner's (submucosal) plexus** is located in the submucosa and mainly controls **glandular secretion**, local blood flow, and absorption, rather than muscle motility. - While it subtly influences motility through local reflexes, it is not the primary controller of peristalsis. *Vagus nerve* - The **vagus nerve (cranial nerve X)** provides parasympathetic innervation to the small intestine, modulating activity but not directly initiating or solely controlling peristalsis. - It influences the activity of the enteric nervous system (including the myenteric plexus) but does not itself generate the complex, coordinated patterns of muscle contraction. *Parasympathetic system* - The **parasympathetic nervous system**, through nerves like the vagus, generally **stimulates gastrointestinal motility**, but it acts by modulating the intrinsic enteric nervous system. - The local control and generation of specific peristaltic movements are primarily mediated by the enteric nervous system, especially the myenteric plexus.
Question 527: Lower esophageal sphincter pressure is increased by all of the following substances, EXCEPT:
- A. Motilin
- B. Gastrin
- C. Substance P
- D. Secretin (Correct Answer)
Explanation: ***Secretin*** - **Secretin** is a gastrointestinal hormone that *decreases* lower esophageal sphincter (LES) pressure - This hormone is released from S cells in the duodenum in response to acidic chyme - Its primary role is to stimulate the pancreas to release **bicarbonate-rich fluid** to neutralize acidic chyme entering the duodenum - By decreasing LES pressure, it facilitates the passage of gastric contents into the duodenum during digestion *Gastrin* - **Gastrin** is a hormone that *increases* lower esophageal sphincter (LES) pressure - This helps prevent gastroesophageal reflux when the stomach is distended - It also stimulates the secretion of **gastric acid** by parietal cells in the stomach - Released from G cells in the gastric antrum in response to protein ingestion *Motilin* - **Motilin** is a peptide hormone that *increases* lower esophageal sphincter (LES) pressure - It initiates the **migrating motor complex (MMC)** during the interdigestive period - Stimulates gastric and intestinal motility - Released from M cells in the duodenum and jejunum *Substance P* - **Substance P** is a neuropeptide that *increases* lower esophageal sphincter (LES) pressure - Functions as both a neurotransmitter and neuromodulator in the enteric nervous system - Plays a role in **smooth muscle contraction** and gastrointestinal motility - Also involved in pain transmission and inflammatory responses
Question 528: Cholecystokinin is produced from:
- A. Hepatocyte
- B. Gastric mucosa
- C. Duodenal mucosa (Correct Answer)
- D. Epithelial cells of distal common bile duct
Explanation: ***Duodenal mucosa*** - **Cholecystokinin (CCK)** is primarily secreted by **I cells**, which are specialized enteroendocrine cells located in the **mucosa of the duodenum** and jejunum. - The release of CCK is stimulated by the presence of **fatty acids** and **amino acids** in the small intestine. *Hepatocyte* - **Hepatocytes** are the main functional cells of the liver, responsible for bile production, metabolism, and detoxification. - They **do not produce regulatory hormones** like cholecystokinin. *Gastric mucosa* - The **gastric mucosa** primarily produces **gastrin**, hydrochloric acid, and pepsinogen, which are involved in gastric digestion. - It does **not secrete cholecystokinin**, which is involved in stimulating gallbladder contraction and pancreatic enzyme release. *Epithelial cells of distal common bile duct* - The **epithelial cells of the common bile duct** are involved in bile transport and modification, but **not in hormone production**. - Their primary role is to line the duct and contribute to the composition of bile.
Question 529: Secretion of bile out of hepatocytes occurs via?
- A. Passive diffusion
- B. Facilitated diffusion
- C. Osmosis
- D. Active transport (Correct Answer)
Explanation: ***Active transport*** - Bile secretion by hepatocytes is an **energy-dependent process** against concentration gradients, requiring specific transporter proteins. - This active mechanism ensures efficient bile flow and prevents reflux, crucial for processes like **fat digestion and absorption**. *Passive diffusion* - **Passive diffusion** involves substances moving down their concentration gradient without energy expenditure. - Bile components, like bile salts, are highly concentrated within hepatocytes and need to be moved against this gradient. *Facilitated diffusion* - This type of diffusion utilizes **transmembrane proteins** to move substances down their concentration gradient, but still does not directly consume ATP. - Bile components are often transported against their concentration gradient, which is not facilitated diffusion. *Osmosis* - **Osmosis** specifically refers to the movement of water across a semipermeable membrane from a region of higher water concentration to a region of lower water concentration. - While water is a component of bile, the primary mechanism of moving bile solutes out of the hepatocyte is not osmosis.
Question 530: What is the primary hormonal action of cholecystokinin (CCK) in response to fat and protein in the duodenum?
- A. Stimulates gallbladder contraction
- B. Inhibits gastric emptying
- C. Increases gastric acid secretion
- D. Stimulates pancreatic enzyme secretion (Correct Answer)
Explanation: ***Stimulates pancreatic enzyme secretion*** - **Cholecystokinin (CCK)** is released by **I cells** in the duodenum and jejunum in response to **fat and protein** in the small intestine. - CCK's primary actions include stimulating **pancreatic enzyme secretion** (amylase, lipase, proteases) and **gallbladder contraction** to release bile. - Both actions work synergistically to digest fats and proteins, making CCK essential for **enzymatic digestion and fat emulsification**. *Stimulates gallbladder contraction* - This is indeed a **major primary function** of CCK (cholecysto = gallbladder, kinin = movement). - CCK causes gallbladder contraction to release stored bile for fat emulsification. - Both pancreatic enzyme secretion and gallbladder contraction are co-equal primary functions; in exam context, pancreatic enzyme secretion is often prioritized as it reflects the broader digestive enzyme role. *Inhibits gastric emptying* - CCK does slow gastric emptying, but this is a **secondary regulatory effect** rather than a primary hormonal action. - This allows time for proper digestion of fats and proteins in the small intestine. *Increases gastric acid secretion* - **Incorrect.** CCK actually has an **inhibitory effect** on gastric acid secretion. - **Gastrin** is the primary hormone that increases gastric acid secretion from parietal cells.
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