Gastrointestinal System — MCQs

Gastrointestinal System Indian Medical PG Practice Questions and MCQs

Question 161: Which of the following statements is true about protein digestion?

A. Protein digestion starts with salivary action.
B. Trypsinogen is converted to trypsin by the action of bicarbonate in the duodenum.
C. Trypsin and chymotrypsin are exopeptidases.
D. 10%-20% protein digestion occurs by pepsin action in the stomach. (Correct Answer)

Explanation: **Explanation:** Protein digestion is a multi-step process involving gastric and pancreatic enzymes. **Why Option D is Correct:** Protein digestion begins in the stomach, not the mouth. Gastric chief cells secrete **pepsinogen**, which is converted to its active form, **pepsin**, by the acidic environment (HCl). Pepsin is an endopeptidase that specifically targets collagen, a major constituent of connective tissue in meat. In the stomach, pepsin is responsible for the initial breakdown of approximately **10%–20% of dietary proteins** into smaller peptides and peptones. **Analysis of Incorrect Options:** * **Option A:** Incorrect. Saliva contains α-amylase (for carbohydrates) and lingual lipase (for fats), but it lacks proteolytic enzymes. Protein digestion begins only upon reaching the stomach. * **Option B:** Incorrect. Trypsinogen is converted into active trypsin by the enzyme **Enteropeptidase (Enterokinase)**, which is secreted by the duodenal brush border. Bicarbonate neutralizes gastric acid but does not activate enzymes. * **Option C:** Incorrect. Trypsin, chymotrypsin, and elastase are **endopeptidases** (they cleave internal peptide bonds). **Exopeptidases**, such as carboxypeptidases, cleave amino acids from the ends of the peptide chains. **High-Yield NEET-PG Pearls:** * **Pepsin's pH Optimum:** It is most active at pH 2.0–3.0 and becomes inactive at pH > 5.0. * **Autocatalysis:** Once a small amount of trypsin is formed by enterokinase, it activates more trypsinogen (autocatalysis) and other zymogens (chymotrypsinogen, procarboxypeptidase). * **Absorption:** Most proteins are absorbed as dipeptides and tripeptides via the **PepT1** transporter, rather than as single amino acids.

Question 162: Parietal cells secrete which of the following substances?

A. Mucus
B. Pepsinogens
C. Gastrin
D. Intrinsic factor (Correct Answer)

Explanation: **Explanation:** Parietal cells (also known as oxyntic cells) are primarily located in the body and fundus of the stomach. Their primary physiological role is the secretion of **Hydrochloric acid (HCl)** and **Intrinsic Factor (IF)**. Intrinsic factor is a glycoprotein essential for the absorption of Vitamin B12 (cobalamin) in the terminal ileum. While HCl is vital for digestion, IF is the only gastric secretion absolutely essential for life. **Analysis of Options:** * **A. Mucus:** Secreted by **Surface mucous cells** and **Mucous neck cells**. It forms a protective barrier against the acidic environment of the stomach. * **B. Pepsinogens:** Secreted by **Chief cells** (Peptic or Zymogenic cells). Pepsinogen is an inactive proenzyme converted to active pepsin by the low pH created by parietal cells. * **C. Gastrin:** Secreted by **G-cells**, which are located in the antrum of the stomach. Gastrin is a hormone that stimulates parietal cells to produce HCl. * **D. Intrinsic Factor:** Correct. It is co-secreted with HCl from the canalicular membranes of parietal cells. **High-Yield Clinical Pearls for NEET-PG:** * **Pernicious Anemia:** An autoimmune destruction of parietal cells leads to a deficiency of Intrinsic Factor, resulting in Vitamin B12 deficiency and megaloblastic anemia. * **Achlorhydria:** The absence of HCl secretion, often seen alongside IF deficiency in chronic atrophic gastritis. * **Stimulants of Parietal Cells:** Gastrin, Acetylcholine (Vagus nerve), and Histamine (via H2 receptors). * **Proton Pump Inhibitors (PPIs):** These drugs act directly on the $H^+/K^+$ ATPase pump located on the apical membrane of parietal cells.

Question 163: Secretion of which of the following is completely independent of vagal stimulation?

A. Pancreas
B. Brunner's gland in duodenum
C. Gall bladder (Correct Answer)
D. Parotid

Explanation: **Explanation:** The correct answer is **Gall bladder (Option C)**. The primary stimulus for gallbladder contraction and the subsequent release of bile is the hormone **Cholecystokinin (CCK)**, which is released from the I-cells of the duodenum in response to fatty acids and amino acids. While the vagus nerve does provide some parasympathetic tone that can cause weak gallbladder contraction during the cephalic phase of digestion, the question asks about "secretion." The gallbladder does not *secrete* bile; it only stores and concentrates it. Furthermore, even in the absence of vagal innervation (e.g., after a vagotomy), gallbladder emptying still occurs effectively via the hormonal action of CCK. **Analysis of Incorrect Options:** * **Pancreas (A):** Pancreatic secretion has a significant **Cephalic Phase** mediated entirely by the vagus nerve. Vagal stimulation triggers the release of enzyme-rich pancreatic juice via acetylcholine. * **Brunner’s Gland (B):** Located in the duodenum, these glands secrete alkaline mucus. Their secretion is increased by both local irritation and **vagal stimulation**. * **Parotid (D):** Salivary secretion is almost entirely under neural control. The parotid gland receives parasympathetic supply via the **Glossopharyngeal nerve (CN IX)**; however, the question likely uses "vagal" as a proxy for parasympathetic/autonomic control in a broader physiological context, or refers to the fact that salivary glands are strictly neurally regulated, unlike the gallbladder which is primarily hormonal. **NEET-PG High-Yield Pearls:** * **CCK Functions:** 1. Gallbladder contraction, 2. Relaxation of the Sphincter of Oddi, 3. Stimulation of pancreatic enzyme secretion, 4. Inhibition of gastric emptying. * **Vagotomy Effect:** A truncal vagotomy leads to gallbladder stasis, which increases the risk of **cholelithiasis** (gallstones) due to the loss of the minor vagal contractile component. * **Brunner's Glands:** They are unique to the duodenum and are inhibited by sympathetic stimulation (which is why stress can lead to duodenal ulcers).

Question 164: Which of the following organs does not exhibit a basic electrical rhythm?

A. Stomach
B. Duodenum
C. Esophagus (Correct Answer)
D. Cecum

Explanation: **Explanation:** The **Basic Electrical Rhythm (BER)**, also known as slow waves, refers to the spontaneous, rhythmic fluctuations in the resting membrane potential of gastrointestinal smooth muscle. These waves are generated by the **Interstitial Cells of Cajal (ICC)**, which act as the electrical pacemakers of the gut. **Why Esophagus is the correct answer:** The esophagus (along with the proximal stomach and the pharynx) does not exhibit BER. The upper third of the esophagus consists of skeletal muscle, while the lower two-thirds consist of smooth muscle. Unlike the rest of the GI tract, esophageal contractions are initiated by the **swallowing reflex** mediated by the vagus nerve (primary peristalsis) or local distension (secondary peristalsis), rather than an intrinsic myogenic rhythm. **Analysis of Incorrect Options:** * **Stomach:** Exhibits a BER of approximately **3 cycles per minute (cpm)**. The pacemaker cells are located in the greater curvature of the corpus (body). * **Duodenum:** Displays the highest frequency of BER in the entire GI tract, at approximately **12 cpm**. * **Cecum/Colon:** The large intestine exhibits BER, though it is less regular than the small intestine. The frequency starts at about **9 cpm** in the cecum and increases toward the sigmoid colon. **High-Yield Facts for NEET-PG:** * **Pacemaker Cells:** Interstitial Cells of Cajal (ICC) located in the myenteric plexus. * **Mechanism:** Slow waves are caused by the cyclic opening of Ca²⁺ channels (depolarization) and K⁺ channels (repolarization). * **Frequency Gradient:** Duodenum (12 cpm) > Ileum (8-9 cpm) > Stomach (3 cpm). * **Clinical Note:** Slow waves do not cause contraction by themselves; they must reach a threshold to trigger **spike potentials** (action potentials), which then lead to muscle contraction.

Question 165: The cephalic phase of gastric secretion is primarily mediated by which system?

A. Parasympathetic nervous system (Correct Answer)
B. Sympathetic nervous system
C. Gastrin
D. Local reflexes

Explanation: ### Explanation The control of gastric secretion is divided into three phases: **Cephalic, Gastric, and Intestinal.** **1. Why the Parasympathetic Nervous System is Correct:** The cephalic phase occurs even before food enters the stomach. It is triggered by the sight, smell, thought, or taste of food. These sensory stimuli activate the cerebral cortex and appetite centers in the amygdala and hypothalamus, which then transmit signals to the **Dorsal Motor Nucleus of the Vagus**. The **Vagus nerve (Parasympathetic)** releases Acetylcholine (ACh), which directly stimulates parietal cells to secrete HCl and triggers G-cells to release gastrin. This phase accounts for approximately **20-30%** of total gastric secretion. **2. Why the Other Options are Incorrect:** * **Sympathetic Nervous System:** Generally inhibits gastrointestinal motility and secretion; it plays no role in the stimulatory cephalic phase. * **Gastrin:** While gastrin is involved in the cephalic phase (via vagal stimulation of G-cells), it is a *hormonal mediator* rather than the primary *system* mediating the phase. The phase is initiated and controlled neurally. * **Local Reflexes:** These are part of the **Gastric phase**, triggered by distension of the stomach wall and the presence of amino acids (mediated by the Myenteric/Enteric nervous system). **3. NEET-PG High-Yield Pearls:** * **Vagus Nerve:** The "master conductor" of the cephalic phase. If the vagus nerve is cut (**Vagotomy**), the cephalic phase is completely abolished. * **Sham Feeding:** A classic experimental method used to study the cephalic phase where food is chewed but not swallowed. * **Gastric Phase:** This is the longest phase, accounting for **60-70%** of total secretion, mediated by both distension (vasovagal reflexes) and chemical stimuli. * **Inhibitory Phase:** The Intestinal phase is primarily inhibitory (via Enterogastrones like Secretin and CCK) to prevent the duodenum from being overwhelmed by acid.

Question 166: Which of the following does not stimulate the enterogastric reflex?

A. Products of protein digestion in the duodenum
B. Duodenal distension
C. H+ ions bathing duodenal mucosa
D. Cholecystokinin (Correct Answer)

Explanation: ### Explanation The **Enterogastric Reflex** is a neural reflex initiated in the duodenum that inhibits gastric motility and secretion. It serves as a "braking mechanism" to prevent the duodenum from being overwhelmed by acidic chyme. **Why Cholecystokinin (CCK) is the correct answer:** The enterogastric reflex is strictly a **neural reflex** mediated by the enteric nervous system, sympathetic fibers, and the vagus nerve. **Cholecystokinin (CCK)**, along with Secretin and Gastric Inhibitory Peptide (GIP), are **hormonal mediators** (often called "enterogastrones"). While CCK does inhibit gastric emptying, it does so via hormonal pathways, not as part of the neural enterogastric reflex arc. **Analysis of Incorrect Options:** * **A. Products of protein digestion:** The presence of peptides and amino acids in the duodenum triggers chemoreceptors that initiate the neural reflex to slow further gastric emptying. * **B. Duodenal distension:** Mechanical stretching of the duodenal wall is the primary physical stimulus for the enterogastric reflex. * **C. H+ ions (Acidity):** Chyme with a pH below 3.5–4.0 strongly stimulates the reflex to protect the duodenal mucosa and allow time for neutralization by pancreatic bicarbonate. **NEET-PG High-Yield Pearls:** * **Stimuli for Enterogastric Reflex:** Distension, Acidity (pH < 4), Hypertonicity, and Irritants in the duodenum. * **Effect:** It inhibits the pyloric pump and increases the tone of the pyloric sphincter. * **Enterogastrone vs. Enterogastric Reflex:** Remember that "Reflex" implies **Neural** (Fast), while "Enterogastrone" implies **Hormonal** (Slower/Sustained). * **CCK Primary Function:** Its most potent stimulus is fat; its primary roles are gallbladder contraction and pancreatic enzyme secretion.

Question 167: Gastric secretions are increased by all of the following except?

A. Somatostatin (Correct Answer)
B. Histamine
C. Vagal stimulation
D. Gastrin

Explanation: **Explanation:** The regulation of gastric acid secretion involves a delicate balance between stimulatory and inhibitory factors. **Why Somatostatin is the correct answer:** Somatostatin is the primary **inhibitory** hormone of the gastrointestinal tract. Produced by **D cells** in the gastric antrum and pancreas, it acts via paracrine and endocrine pathways to decrease gastric acid secretion. It achieves this by directly inhibiting parietal cells and indirectly by suppressing the release of stimulatory hormones like gastrin and histamine. Therefore, it decreases rather than increases secretions. **Analysis of Incorrect Options:** * **Histamine:** Released by Enterochromaffin-like (ECL) cells, it binds to **H2 receptors** on parietal cells, significantly increasing HCl secretion via the cAMP pathway. * **Vagal Stimulation:** The Vagus nerve (Parasympathetic) stimulates acid secretion through two pathways: directly via **Acetylcholine** acting on M3 receptors of parietal cells, and indirectly by stimulating G-cells to release Gastrin. * **Gastrin:** Produced by **G-cells**, it is a potent stimulator of acid secretion. It acts directly on parietal cells and indirectly by triggering histamine release from ECL cells. **NEET-PG High-Yield Pearls:** * **The "Universal Off-Switch":** Somatostatin inhibits almost all GI hormones (Gastrin, Insulin, Glucagon, Secretin, CCK). * **Receptor Pathways:** Gastrin and Acetylcholine use the **Ca²⁺/IP3 pathway**, while Histamine uses the **cAMP pathway**. This synergy is why H2 blockers can reduce the effect of all three stimulants. * **Proton Pump:** The final common pathway for all these stimulants is the **H+/K+ ATPase pump** in the canalicular membrane of the parietal cell.

Question 168: Which one of the following enzymes is secreted by both the exocrine pancreas and the intestinal mucosa?

A. Carboxypeptidase (Correct Answer)
B. Trehalase
C. Elastase
D. Amylase

Explanation: **Explanation:** The correct answer is **Carboxypeptidase**. This enzyme is a protease that cleaves peptide bonds at the carboxyl-terminal end of proteins. It exists in two forms: 1. **Pancreatic Carboxypeptidase:** Secreted by the exocrine pancreas as pro-carboxypeptidase (an inactive zymogen), which is activated by trypsin in the duodenum. 2. **Intestinal Carboxypeptidase:** Produced by the enterocytes of the intestinal mucosa (brush border) to complete the final stages of protein digestion into individual amino acids. **Analysis of Incorrect Options:** * **Trehalase:** This is a disaccharidase found exclusively in the **intestinal brush border**. It is responsible for breaking down trehalose (found in mushrooms). * **Elastase:** This is a protease secreted solely by the **pancreas** as pro-elastase. It specifically digests elastin fibers. (Note: Fecal elastase is a clinical marker for pancreatic sufficiency). * **Amylase:** While amylase is secreted by the **pancreas** (and salivary glands), it is **not** produced by the intestinal mucosa. The intestine produces disaccharidases (like maltase, sucrase, and lactase) to finish carbohydrate digestion, but not amylase itself. **NEET-PG High-Yield Pearls:** * **Activation Cascade:** Remember that **Enterokinase** (secreted by the intestinal mucosa) is the "master switch" that activates pancreatic trypsinogen to trypsin, which then activates all other pancreatic proteases, including pro-carboxypeptidase. * **Endopeptidases vs. Exopeptidases:** Trypsin, chymotrypsin, and elastase are *endopeptidases* (cleave internal bonds). Carboxypeptidase is an *exopeptidase* (cleaves terminal bonds). * **Absorption:** Only monosaccharides, amino acids, and di/tripeptides can be absorbed by the enterocytes. Any enzyme deficiency in the brush border leads to malabsorption syndromes.

Question 169: Antiperistalsis is naturally seen in which part of the digestive tract?

A. Stomach
B. Duodenum
C. Colon (Correct Answer)
D. Antiperistalsis does not occur normally

Explanation: ### Explanation **Correct Answer: C. Colon** **Why it is correct:** Antiperistalsis (reverse peristalsis) is a physiological phenomenon characterized by wave-like contractions moving in an orad (toward the mouth) direction. In the **colon**, specifically the ascending colon and cecum, antiperistaltic waves occur naturally. This serves a critical functional purpose: it slows the transit of chyme, allowing for prolonged contact with the mucosal surface. This maximizes the **absorption of water and electrolytes** and facilitates the mixing of contents to form solid feces. **Why other options are incorrect:** * **Stomach:** Normal gastric motility involves propulsion, grinding, and retropulsion (mixing). While "vomiting" involves reverse movement, it is a pathological reflex, not a routine physiological process of the healthy stomach. * **Duodenum:** Motility here is dominated by segmentation (mixing) and peristalsis (propulsion) to move chyme toward the jejunum. Antiperistalsis here would cause bile reflux into the stomach, which is generally non-physiological. * **Option D:** This is incorrect because antiperistalsis is a well-documented normal feature of colonic motility. **High-Yield Facts for NEET-PG:** * **Pacemaker of the Gut:** The Interstitial Cells of Cajal (ICC) generate the Slow Waves (Basal Electrical Rhythm). * **Gastrocolic Reflex:** Distension of the stomach increases colonic motility (mediated by CCK and gastrin), often leading to the urge to defecate after a meal. * **Mass Movements:** These are modified peristaltic waves that occur 1–3 times daily in the colon to propel feces toward the rectum; they are distinct from the localized antiperistalsis seen in the proximal colon. * **Vomiting Center:** Located in the area postrema of the medulla, it coordinates the pathological reverse movement of gastric contents.

Question 170: Which of the following is NOT a function of gastrin?

A. Stimulation of gastric acid secretion
B. Stimulation of pepsin secretion
C. Stimulation of growth of the gastric mucosa
D. Stimulation of direct cholesterol absorption (Correct Answer)

Explanation: **Explanation:** Gastrin is a peptide hormone primarily secreted by **G-cells** in the antrum of the stomach and the duodenum. Its primary role is the regulation of gastric function and mucosal integrity. **Why Option D is correct:** Gastrin has no physiological role in the absorption of lipids or cholesterol. Cholesterol absorption is a complex process occurring in the small intestine involving bile acid micelle formation and specific transporters like **NPC1L1**. Gastrin’s functions are localized primarily to the stomach and proximal duodenum. **Analysis of Incorrect Options:** * **A. Stimulation of gastric acid secretion:** This is the primary function of gastrin. It stimulates parietal cells both directly and indirectly (by triggering **Enterochromaffin-like (ECL) cells** to release histamine), leading to H+ secretion. * **B. Stimulation of pepsin secretion:** Gastrin increases the secretion of pepsinogen from **Chief cells**, which is then converted to pepsin in the acidic environment of the stomach. * **C. Stimulation of growth of gastric mucosa:** Gastrin exerts a potent **trophic effect** on the gastric mucosa (excluding the antrum) and the colonic mucosa. Hypergastrinemia (as seen in Zollinger-Ellison Syndrome) leads to marked hyperplasia of the gastric folds. **NEET-PG High-Yield Pearls:** * **Stimuli for Gastrin:** Distension of the stomach, presence of amino acids/peptides (Phenylalanine and Tryptophan are most potent), and **Vagal stimulation** (via Gastrin-Releasing Peptide/GRP). * **Inhibition:** Gastrin secretion is inhibited by a luminal pH < 1.5 and by **Somatostatin**. * **Zollinger-Ellison Syndrome:** A gastrin-secreting tumor (Gastrinoma) characterized by multiple peptic ulcers, gastric mucosal hypertrophy, and steatorrhea (due to low pH inactivating pancreatic lipase).

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