Gastrointestinal System — MCQs

Gastrointestinal System Indian Medical PG Practice Questions and MCQs

Question 261: Chyme is propelled forward in the small intestine by which mechanism?

A. Segmentation
B. Haustrations
C. Migratory motor complex
D. Peristalsis (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Peristalsis**. **1. Why Peristalsis is Correct:** Peristalsis is the primary propulsive movement of the gastrointestinal tract. It is characterized by a "law of the gut" where a contractile ring forms behind the bolus (proximal side) while the segment ahead (distal side) relaxes. This coordinated reflex, mediated by the **Myenteric (Auerbach’s) plexus**, ensures the forward movement of chyme toward the anus. In the small intestine, these waves are typically weak and travel short distances to allow for adequate nutrient absorption. **2. Why Other Options are Incorrect:** * **A. Segmentation:** This is the most common movement in the small intestine, but it is **non-propulsive**. Its primary function is mixing chyme with digestive juices and increasing contact with the mucosa for absorption. * **B. Haustrations:** These are characteristic "sac-like" mixing movements found in the **large intestine (colon)**, not the small intestine. They are analogous to segmentation but involve the contraction of *taenia coli*. * **C. Migratory Motor Complex (MMC):** While propulsive, the MMC occurs only during the **inter-digestive (fasting) state**. It acts as a "housekeeper" to clear residual debris and bacteria. Peristalsis is the mechanism used during the digestive phase. **High-Yield NEET-PG Pearls:** * **Gastroileal Reflex:** Increased gastric activity causes increased peristalsis in the ileum and relaxation of the ileocecal sphincter (mediated by Gastrin and ANS). * **Peristaltic Rush:** Intense, rapid peristalsis caused by severe mucosal irritation (e.g., infectious diarrhea). * **Key Neurotransmitters:** **ACh and Substance P** cause contraction (behind bolus); **NO and VIP** cause relaxation (ahead of bolus).

Question 262: Which of the following enzymes decreases gastric motility?

A. Secretin
B. CCK-PZ
C. Both secretin and CCK-PZ (Correct Answer)
D. None of the above

Explanation: **Explanation:** The regulation of gastric motility is a coordinated process involving neural and hormonal signals. The correct answer is **Both secretin and CCK-PZ** because both hormones act as "enterogastrones"—hormones released by the upper small intestine that inhibit gastric functions to ensure optimal digestion and absorption. **1. Why the correct answer is right:** * **Secretin:** Released by S-cells in the duodenum in response to acidic chyme (low pH). Its primary role is to stimulate pancreatic bicarbonate secretion, but it also inhibits gastric acid secretion and slows down gastric emptying (motility) to allow the neutralization of acid. * **CCK-PZ (Cholecystokinin-Pancreozymin):** Secreted by I-cells in the duodenum and jejunum in response to fat and protein breakdown products. CCK slows gastric emptying to ensure that the small intestine has sufficient time to emulsify and digest fats. **2. Why other options are incorrect:** * **Option A & B:** While both individually decrease motility, selecting only one would be incomplete. In the context of NEET-PG, when multiple hormones perform a similar physiological function, the "Both" option is the most accurate choice. **High-Yield Clinical Pearls for NEET-PG:** * **Enterogastric Reflex:** This is the neural counterpart to these hormones; distension of the duodenum inhibits gastric motility via the vagus nerve. * **Gastrin:** Unlike Secretin and CCK, Gastrin *increases* gastric motility and acid secretion. * **GIP (Gastric Inhibitory Peptide):** Also an enterograstrone that inhibits motility, though its primary physiological role is stimulating insulin release (Incretin effect). * **Mnemonic:** "S-I-G" (Secretin, I-cells/CCK, GIP) are the primary inhibitors of the stomach.

Question 263: Gastric secretion is stimulated by all of the following except?

A. Vagus nerve
B. Gastrin
C. Secretin (Correct Answer)
D. Gastric distention

Explanation: **Explanation:** The regulation of gastric acid secretion involves a balance between stimulatory and inhibitory factors. **Why Secretin is the correct answer:** Secretin is an **enterogastrone**—a hormone released by the S-cells of the duodenum in response to acidic chyme (pH < 4.5). Its primary role is to inhibit gastric acid secretion and gastric emptying while stimulating the release of bicarbonate-rich pancreatic juice. By inhibiting the parietal cells and the release of Gastrin, it acts as a "brake" on the stomach to protect the duodenum from acid injury. **Why the other options are incorrect:** * **Vagus Nerve:** Stimulates acid secretion via the release of Acetylcholine (ACh), which acts directly on parietal cells (M3 receptors) and indirectly by stimulating G-cells to release Gastrin. * **Gastrin:** The most potent hormonal stimulator of acid secretion. It is released from G-cells in the antrum and acts via CCK-B receptors on parietal cells and Enterochromaffin-like (ECL) cells (releasing histamine). * **Gastric Distention:** This is a mechanical trigger during the **Gastric Phase** of secretion. Distention activates long (vagovagal) and short (myenteric) reflexes that stimulate G-cells and parietal cells. **High-Yield Clinical Pearls for NEET-PG:** * **The "Big Three" Stimulants:** Acetylcholine (Neurocrine), Gastrin (Hormonal), and Histamine (Paracrine). * **Potentiation:** The combined effect of these three stimulants is greater than the sum of their individual effects. * **Other Inhibitors:** Somatostatin (the universal inhibitor), GIP (Gastric Inhibitory Peptide), and CCK also inhibit gastric acid secretion. * **Receptor Match:** ACh → M3; Gastrin → CCK-B; Histamine → H2.

Question 264: Which of the following plasma proteins is not synthesized primarily in the liver?

A. Angiotensinogen
B. C-reactive protein
C. Angiotensin converting enzyme (Correct Answer)
D. Fibrinogen

Explanation: **Explanation:** The liver is the primary metabolic factory of the body, responsible for synthesizing the vast majority of plasma proteins, including albumin, coagulation factors, and acute-phase reactants [2]. **Why Angiotensin Converting Enzyme (ACE) is the correct answer:** Unlike most circulating proteins, **ACE is primarily synthesized by the vascular endothelial cells**, with the highest concentration found in the **pulmonary capillaries (lungs)**. It is also found in the brush border of the proximal convoluted tubules in the kidneys. While it circulates in the plasma, its origin is endothelial rather than hepatic. **Analysis of Incorrect Options:** * **Angiotensinogen:** This is an $\alpha$-2 globulin produced and constitutively secreted into the plasma exclusively by the **liver** [1]. It is the essential substrate for Renin. * **C-reactive protein (CRP):** This is a classic "acute-phase reactant." Its synthesis is induced in the **liver** in response to inflammatory cytokines like Interleukin-6 (IL-6). * **Fibrinogen:** This is Factor I of the coagulation cascade [2]. All coagulation factors (except Factor VIII and von Willebrand Factor) are synthesized primarily in the **liver**. **High-Yield NEET-PG Pearls:** * **Liver Protein Synthesis:** The liver produces all plasma proteins *except* **Immunoglobulins** (produced by plasma cells) and **von Willebrand Factor** (produced by endothelial cells and megakaryocytes) [2]. * **ACE Inhibitors:** Drugs like Enalapril act on the ACE found in lung endothelium, which is why a common side effect is a dry cough (due to the accumulation of bradykinin in the lungs). * **Albumin:** The most abundant plasma protein, synthesized solely by the liver [3]; it is the best indicator of chronic liver synthetic function.

Question 265: Production of bile takes place in:

A. The gall bladder
B. Kupffer cells
C. Hepatic duct
D. Hepatocytes (Correct Answer)

Explanation: **Explanation:** **1. Why Hepatocytes are the Correct Answer:** Bile production is a primary exocrine function of the liver. It is synthesized continuously by the **hepatocytes** (the functional cells of the liver). These cells secrete bile into tiny canals called bile canaliculi, which eventually drain into the hepatic ducts. Bile consists of bile salts, bilirubin, cholesterol, lecithin, and electrolytes. **2. Analysis of Incorrect Options:** * **A. The Gallbladder:** This is a common misconception. The gallbladder **does not produce** bile; it only stores and concentrates bile (up to 10-20 fold) by absorbing water and electrolytes. * **B. Kupffer Cells:** These are specialized macrophages located in the sinusoids of the liver. Their primary role is phagocytosis of pathogens and old red blood cells; they are part of the reticuloendothelial system, not the secretory system. * **C. Hepatic Duct:** This is a part of the biliary tree that serves as a **conduit** or passage for bile to travel from the liver to the gallbladder or duodenum. It has no role in the synthesis of bile. **3. NEET-PG High-Yield Pearls:** * **Daily Secretion:** The liver produces approximately **500–1000 ml** of bile per day. * **Bile Salts:** These are the only components of bile that provide a digestive function (emulsification of fats). They are derived from **cholesterol**. * **Rate-Limiting Step:** The conversion of cholesterol to bile acids is regulated by the enzyme **7-alpha-hydroxylase**. * **Enterohepatic Circulation:** About 95% of bile salts are reabsorbed in the **terminal ileum** and returned to the liver via the portal vein.

Question 266: Which hormone is secreted by I cells?

A. Secretin
B. Gastrin
C. CCK (Correct Answer)
D. Motilin

Explanation: **Explanation:** The correct answer is **CCK (Cholecystokinin)**. CCK is a peptide hormone synthesized and secreted by the **I cells** located primarily in the mucosal epithelium of the duodenum and jejunum. Its secretion is stimulated by the presence of fatty acids and amino acids in the duodenum. CCK acts to stimulate gallbladder contraction (releasing bile) and pancreatic enzyme secretion, while simultaneously slowing gastric emptying to ensure optimal digestion. **Analysis of Incorrect Options:** * **A. Secretin:** Secreted by **S cells** of the duodenum. Its primary role is to stimulate the secretion of bicarbonate-rich pancreatic fluid to neutralize gastric acid. * **B. Gastrin:** Secreted by **G cells** located in the antrum of the stomach and the duodenum. It primarily stimulates gastric acid (HCl) secretion from parietal cells. * **D. Motilin:** Secreted by **M cells** of the upper small intestine. It is responsible for the Migrating Motor Complex (MMC), which clears the gut of debris during the fasting state. **High-Yield Clinical Pearls for NEET-PG:** * **CCK and Satiety:** CCK acts on the hypothalamus to induce a feeling of fullness (satiety). * **Diagnostic Use:** CCK can be used in a HIDA scan to assess gallbladder ejection fraction. * **Potentiation:** CCK and Secretin work synergistically; CCK potentiates the bicarbonate-secreting effects of Secretin. * **Mnemonic:** Remember **"I see (C) CCK"** and **"S for Secretin."**

Question 267: Acid reflux is prevented by all mechanisms except?

A. A long intra-abdominal esophagus
B. Increased intra-abdominal pressure
C. The right crus of the diaphragm
D. Increased intra-thoracic pressure (Correct Answer)

Explanation: **Explanation:** The prevention of gastroesophageal reflux (GERD) depends on the **Anti-reflux Barrier**, which maintains a pressure gradient where the Lower Esophageal Sphincter (LES) pressure remains higher than the intragastric pressure. **Why "Increased intra-thoracic pressure" is the correct answer:** The esophagus is located within the thoracic cavity, where the pressure is normally sub-atmospheric (negative). An **increase** in intra-thoracic pressure (e.g., during coughing or heavy lifting) actually creates a "suction" effect or reduces the pressure gradient that keeps the LES closed, potentially facilitating reflux. Therefore, it does not prevent reflux; rather, it can predispose to it. **Why the other options are incorrect (Mechanisms that PREVENT reflux):** * **A. Long intra-abdominal esophagus:** Approximately 2–4 cm of the esophagus lies below the diaphragm. This segment is exposed to positive intra-abdominal pressure, which compresses the esophageal walls, acting as a "flap-valve" to prevent gastric contents from entering. * **B. Increased intra-abdominal pressure:** When abdominal pressure rises (e.g., during pregnancy or Valsalva), it simultaneously compresses the intra-abdominal segment of the esophagus. This ensures the sphincter stays closed even when the stomach is under pressure. * **C. Right crus of the diaphragm:** The esophagus passes through the esophageal hiatus formed by the right crus. It acts as an **extrinsic sphincter** (the "pinch-cock" mechanism), contracting during inspiration to prevent reflux when the pressure gradient is highest. **High-Yield Clinical Pearls for NEET-PG:** * **Angle of His:** The acute angle between the esophagus and the fundus of the stomach acts as a mechanical valve to prevent reflux. * **Phrenoesophageal ligament:** Anchors the esophagus to the diaphragm, maintaining the position of the LES. * **Z-line:** The squamocolumnar junction; its displacement cranially is a hallmark of Barrett’s Esophagus. * **Hormonal influence:** Gastrin increases LES tone, while Secretin, CCK, and Progesterone (as seen in pregnancy) decrease it.

Question 268: Complex polysaccharides are converted to glucose and absorbed with the help of which of the following?

A. Na+K+ATPase
B. Sucrase (Correct Answer)
C. Enterokinase
D. Carboxypeptidase

Explanation: **Explanation:** The digestion of complex polysaccharides (like starch and glycogen) begins in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase. These enzymes break down large chains into disaccharides (sucrose, lactose, maltose) and oligosaccharides. **1. Why Sucrase is Correct:** The final stage of carbohydrate digestion occurs at the **brush border** of the small intestinal mucosa. **Sucrase** is a brush border enzyme (disaccharidase) that hydrolyzes sucrose into glucose and fructose. Since the question asks about the conversion to glucose for absorption, sucrase is the specific enzyme responsible for this final hydrolytic step, allowing monosaccharides to be transported into the enterocyte. **2. Why other options are incorrect:** * **Na+K+ATPase:** This is a pump located on the basolateral membrane. While it creates the sodium gradient necessary for glucose absorption (via SGLT-1), it does not "convert" polysaccharides into glucose. * **Enterokinase (Enteropeptidase):** This enzyme is responsible for converting trypsinogen into active trypsin. It is involved in protein digestion, not carbohydrate metabolism. * **Carboxypeptidase:** This is a pancreatic exopeptidase that cleaves peptide bonds at the carboxyl end of proteins. It is essential for protein digestion. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The absorption of carbohydrates is limited by the rate of brush border hydrolysis, not the transport across the membrane. * **SGLT-1:** Transports Glucose and Galactose via secondary active transport (Sodium-dependent). * **GLUT-5:** Specifically transports Fructose via facilitated diffusion. * **GLUT-2:** The common transporter for all three monosaccharides (Glucose, Galactose, Fructose) to exit the basolateral membrane into the blood.

Question 269: Motilin is secreted by cells in which part of the gastrointestinal tract?

A. Stomach
B. Duodenum/ileum (Correct Answer)
C. Pancreas
D. Liver

Explanation: **Explanation:** **Motilin** is a 22-amino acid polypeptide hormone primarily responsible for the regulation of the **Migrating Motor Complex (MMC)**. 1. **Why the correct answer is right:** Motilin is secreted by specialized enteroendocrine cells called **Mo cells** (or M cells). These cells are predominantly located in the mucosal epithelium of the **upper small intestine**, specifically the **duodenum and jejunum** (and to a lesser extent, the ileum). Its secretion occurs cyclically during the fasting state (every 90–120 minutes) to stimulate "housekeeping" contractions that clear the gut of undigested debris. 2. **Why the incorrect options are wrong:** * **Stomach:** While the stomach is the target organ where Motilin initiates contractions, it is not the primary site of synthesis. The stomach primarily secretes Gastrin (G cells) and Ghrelin (P/D1 cells). * **Pancreas:** The pancreas is an endocrine/exocrine organ secreting Insulin, Glucagon, and Somatostatin, but it does not produce Motilin. * **Liver:** The liver produces bile and plasma proteins but has no role in the synthesis of gastrointestinal peptide hormones like Motilin. **High-Yield Clinical Pearls for NEET-PG:** * **Erythromycin Connection:** Erythromycin acts as a **Motilin agonist**. It binds to motilin receptors and is clinically used to treat **gastroparesis** (diabetic or post-operative) to stimulate gastric emptying. * **The "Housekeeper" Hormone:** Motilin levels are highest during **fasting** and are inhibited immediately upon ingestion of food. * **MMC Phases:** Motilin is responsible for initiating **Phase III** of the Migrating Motor Complex (the phase of maximal contraction).

Question 270: Which of the following is absorbed in the colon?

A. Iron
B. Proteins
C. Bile salts
D. Electrolytes (Correct Answer)

Explanation: **Explanation:** The colon (large intestine) serves as the final site for the absorption of water and electrolytes, transforming liquid chyme into solid feces. While the majority of nutrient absorption occurs in the small intestine, the colon is highly efficient at absorbing **sodium (Na⁺)** and **chloride (Cl⁻)**. Sodium is actively absorbed via epithelial sodium channels (ENaC), creating an osmotic gradient that facilitates the passive absorption of water. The colon also secretes potassium and bicarbonate, making it a key site for acid-base and electrolyte balance. **Analysis of Incorrect Options:** * **A. Iron:** Primarily absorbed in the **duodenum** and upper jejunum. It requires an acidic environment for optimal absorption in its ferrous (Fe²⁺) state. * **B. Proteins:** Digested into amino acids, dipeptides, and tripeptides, which are absorbed almost exclusively in the **small intestine** (duodenum and jejunum). * **C. Bile Salts:** While a small amount undergoes passive diffusion earlier, 95% of bile salts are actively reabsorbed in the **terminal ileum** via the enterohepatic circulation. **High-Yield NEET-PG Pearls:** * **Maximum Absorptive Capacity:** The colon can absorb about 5–8 liters of fluid and electrolytes per day. If this capacity is exceeded, diarrhea occurs. * **Aldosterone Effect:** Aldosterone increases sodium absorption and potassium secretion in the distal colon, similar to its action on the renal distal tubules. * **Short-Chain Fatty Acids (SCFAs):** The colon also absorbs SCFAs (like butyrate), which are produced by bacterial fermentation of unabsorbed carbohydrates and serve as the primary energy source for colonocytes.

Practice by Chapter

Gastrointestinal Motility

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Gastrointestinal Secretions

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Digestion and Absorption

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Gastrointestinal Hormones

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Pancreatic Exocrine Function

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Gastrointestinal Circulation

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Intestinal Immune System

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Gut Microbiome

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Regulation of Food Intake

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