Gastrointestinal System — MCQs

Gastrointestinal System Indian Medical PG Practice Questions and MCQs

Question 411: Which of the following materials can be absorbed directly by the surface lining cells of the stomach?

A. Vitamin B12
B. Polysaccharides
C. Triglycerides
D. Alcohol (Correct Answer)

Explanation: **Explanation:** The stomach is primarily an organ of digestion and storage rather than absorption. Its mucosal lining is covered by a thick layer of protective mucus and lacks the specialized villi and microvilli found in the small intestine, which are essential for high-capacity absorption. **Why Alcohol is the Correct Answer:** Alcohol (Ethanol) is **lipid-soluble** and has a low molecular weight. These properties allow it to diffuse directly across the gastric mucosal barrier into the systemic circulation. Approximately 20% of ingested alcohol is absorbed in the stomach, while the remaining 80% is absorbed in the small intestine. This rapid gastric absorption explains why alcohol's effects are felt quickly, especially on an empty stomach. **Analysis of Incorrect Options:** * **Vitamin B12:** It requires **Intrinsic Factor (IF)**, secreted by gastric parietal cells, for its eventual absorption. However, the actual absorption occurs in the **terminal ileum**, not the stomach. * **Polysaccharides:** These are complex carbohydrates (e.g., starch). They must be broken down into monosaccharides (glucose, galactose, fructose) by salivary and pancreatic amylase before they can be absorbed in the **small intestine**. * **Triglycerides:** These are large lipid molecules that require emulsification by bile salts and enzymatic cleavage by lipases into fatty acids and monoglycerides. Their absorption occurs exclusively in the **small intestine**. **High-Yield Clinical Pearls for NEET-PG:** * **Gastric Absorptive Capacity:** Besides alcohol, only a few substances are absorbed by the stomach, notably **Aspirin (NSAIDs)** and certain lipid-soluble drugs. * **Aspirin Toxicity:** At low gastric pH, aspirin remains non-ionized and lipid-soluble, allowing it to enter gastric cells where it can cause mucosal damage (gastritis). * **Water:** Small amounts of water can be absorbed, but this is physiologically insignificant compared to the small intestine.

Question 412: In which form are the majority of carbohydrates absorbed by the small intestine?

A. Polysaccharide
B. Disaccharide
C. Monosaccharide (Correct Answer)
D. Pentose sugar

Explanation: **Explanation:** The primary site for carbohydrate digestion and absorption is the small intestine. The correct answer is **Monosaccharides** because the intestinal mucosa can only transport simple sugars across the enterocyte membrane. **1. Why Monosaccharides are correct:** Dietary carbohydrates (starches and sugars) must be broken down into their simplest forms—**Glucose, Galactose, and Fructose**—before absorption. This is achieved through the action of salivary and pancreatic amylases, followed by "brush border enzymes" (lactase, sucrase, maltase) located on the microvilli. Once converted to monosaccharides, they are absorbed via specific transporters: * **SGLT-1:** Transports Glucose and Galactose (Secondary active transport with Sodium). * **GLUT-5:** Transports Fructose (Facilitated diffusion). * **GLUT-2:** Transports all three into the portal circulation. **2. Why other options are incorrect:** * **Polysaccharides (e.g., Starch, Glycogen):** These are large polymers. The body lacks a mechanism to transport these bulky molecules across the cell membrane; they must first be hydrolyzed. * **Disaccharides (e.g., Lactose, Sucrose, Maltose):** While these are common in our diet, they cannot be absorbed intact. Deficiency of brush border enzymes (like Lactase) leads to these sugars remaining in the lumen, causing osmotic diarrhea (Lactose Intolerance). * **Pentose Sugars:** While some pentoses (like ribose) are absorbed, the "majority" of dietary carbohydrates are hexoses (6-carbon sugars) derived from starch. **High-Yield Facts for NEET-PG:** * **Rate-limiting step:** The absorption of carbohydrates is limited by the rate of mucosal uptake, not by the rate of digestion. * **SGLT-1 Importance:** This transporter is the physiological basis for **Oral Rehydration Solution (ORS)**; sodium absorption is enhanced by the presence of glucose. * **Location:** Most carbohydrate absorption is completed by the time the chyme reaches the mid-jejunum.

Question 413: Which of the following does not simulate the enterogastric reflex?

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

Explanation: ### Explanation The **Enterogastric Reflex** is a purely **neural reflex** (part of the enterogastric inhibitory mechanism) that inhibits gastric motility and secretion to prevent the duodenum from being overloaded. It is mediated by the enteric nervous system, extrinsic sympathetic fibers, and the vagus nerve. **Why "Hormones" is the correct answer:** While hormones like Secretin, Cholecystokinin (CCK), and Gastric Inhibitory Peptide (GIP) do inhibit gastric emptying, they are classified as **Enterogastrones** (humoral control), not part of the **Enterogastric Reflex** (neural control). The question specifically asks for the triggers of the *reflex* mechanism. **Analysis of Incorrect Options:** * **A. Products of protein digestion:** The presence of breakdown products (especially peptides and amino acids) in the duodenum triggers sensory receptors that initiate the neural reflex to slow further gastric emptying. * **B. Duodenal distension:** Mechanical stretching of the duodenal wall is the primary physical trigger for the enterogastric reflex. * **C. H+ ions (Acidity):** High acidity (pH < 3.5–4.0) in the duodenal chyme stimulates the reflex to protect the duodenal mucosa and allow time for pancreatic bicarbonate to neutralize the acid. **NEET-PG High-Yield Pearls:** * **Stimuli for the Reflex:** Distension, Acidity (H+), Hypertonicity (osmolarity), and Irritants/Protein breakdown products in the duodenum. * **Effect:** It inhibits the pyloric pump and increases the tone of the pyloric sphincter. * **Distinction:** Remember the "Rule of Two": Gastric emptying is regulated by **Neural** (Enterogastric reflex) and **Hormonal** (Enterogastrones like CCK/Secretin) pathways. They work together, but they are distinct mechanisms.

Question 414: Which of the following factors in bile juice is responsible for preventing the precipitation of cholesterol and formation of gallstones?

A. High alkaline condition
B. High concentration of bicarbonates
C. Bile salts (Correct Answer)
D. Bile pigments

Explanation: ### Explanation **Why Bile Salts are the Correct Answer:** Cholesterol is a lipid that is virtually insoluble in water. In bile, it is kept in a stable, soluble state through the formation of **mixed micelles**. Bile salts (along with lecithin/phospholipids) are amphipathic molecules; they surround the hydrophobic cholesterol molecules, orienting their hydrophilic heads outward. This prevents cholesterol from crystallizing. If the ratio of bile salts to cholesterol decreases (due to decreased bile acid synthesis or increased cholesterol secretion), the bile becomes **supersaturated**, leading to the precipitation of cholesterol crystals and the formation of gallstones (cholelithiasis). **Analysis of Incorrect Options:** * **A & B (High alkaline condition/Bicarbonates):** While bile is alkaline and contains bicarbonates (stimulated by Secretin) to neutralize gastric acid in the duodenum, these factors do not directly influence the solubility of cholesterol. They primarily provide an optimal pH for pancreatic enzyme function. * **D (Bile pigments):** Bile pigments (mainly bilirubin) are waste products of hemoglobin breakdown. While they can form "pigment stones" (calcium bilirubinate) in conditions like chronic hemolysis, they do not prevent cholesterol precipitation; in fact, excess bilirubin contributes to a different type of stone formation. **High-Yield Clinical Pearls for NEET-PG:** * **The Solubility Triangle (Admirand’s Pyramid):** This diagram illustrates the critical ratio between bile salts, lecithin, and cholesterol required to maintain a micellar liquid state. * **Enterohepatic Circulation:** 95% of bile salts are reabsorbed in the **terminal ileum**. Resection or disease of the terminal ileum (e.g., Crohn’s disease) leads to bile salt malabsorption and a high risk of cholesterol gallstones. * **Lecithin:** It is the primary phospholipid in bile that works synergistically with bile salts to solubilize cholesterol.

Question 415: Parasympathetic stimulation induces salivary acinar cells to release which protease?

A. Bradykinin
B. Kallikrein (Correct Answer)
C. Kininogen
D. Kinin

Explanation: **Explanation:** Salivary secretion is primarily regulated by the autonomic nervous system. Parasympathetic stimulation (via the facial and glossopharyngeal nerves) triggers the acinar cells to secrete not only water and electrolytes but also specific enzymes. **Why Kallikrein is correct:** When parasympathetic nerves are stimulated, acinar cells release the enzyme **Kallikrein**. Once in the interstitial fluid, Kallikrein acts as a protease, cleaving the plasma protein **alpha-2-globulin (kininogen)** to form **bradykinin**. Bradykinin is a potent vasodilator; it increases local blood flow to the salivary glands, providing the necessary fluid and nutrients to sustain high rates of saliva production. **Analysis of Incorrect Options:** * **A. Bradykinin:** This is a vasodilator peptide, not a protease. It is the *product* of the reaction catalyzed by Kallikrein, not the enzyme released by the acinar cells. * **C. Kininogen:** This is the precursor substrate (a plasma protein) found in the blood/interstitial fluid, not an enzyme released by the salivary cells. * **D. Kinin:** This is a general term for a group of polypeptides (like bradykinin and kallidin) that cause vasodilation. It is not a specific protease. **NEET-PG High-Yield Pearls:** * **Dual Innervation:** Unlike most organs where sympathetic and parasympathetic systems are antagonistic, both systems **increase** salivary secretion. However, parasympathetic produces high-volume, watery saliva, while sympathetic produces low-volume, thick, viscous saliva. * **Aldosterone Effect:** Similar to its action in the kidneys, aldosterone acts on salivary ducts to increase Na+ reabsorption and K+ secretion. * **Tonicity:** Saliva is always **hypotonic** compared to plasma, especially at low flow rates, because the ductal cells reabsorb more solute than water.

Question 416: What is the normal bacterial count in the duodenum per gram?

A. 10^5 per gram (Correct Answer)
B. 10^8 per gram
C. 10^10 per gram
D. 10^12 per gram

Explanation: **Explanation:** The distribution of microflora in the human gastrointestinal tract follows a distinct longitudinal gradient, increasing significantly from the stomach to the colon. **1. Why Option A is Correct:** The **duodenum** is considered a relatively sterile environment compared to the lower GI tract. The bacterial count in the duodenum and proximal jejunum typically ranges from **$10^3$ to $10^5$ organisms per gram** of contents. This low concentration is maintained by several protective mechanisms: * **Gastric Acid:** The high acidity of the stomach kills most ingested microorganisms before they reach the duodenum. * **Bile and Pancreatic Enzymes:** These secretions have potent antimicrobial properties. * **Peristalsis:** Rapid forward movement (the Migrating Motor Complex) prevents bacterial colonization and stagnation. **2. Why Other Options are Incorrect:** * **Option B ($10^8$):** This concentration is characteristic of the **distal ileum**, which acts as a transition zone between the sparse upper tract and the dense lower tract. * **Options C and D ($10^{10}$ to $10^{12}$):** These extremely high concentrations are found in the **colon (large intestine)**. The colon houses the bulk of the human microbiome, dominated by anaerobes like *Bacteroides* and *Bifidobacterium*. **High-Yield Clinical Pearls for NEET-PG:** * **SIBO (Small Intestinal Bacterial Overgrowth):** Defined clinically when bacterial counts in the proximal small intestine exceed **$>10^5$ CFU/mL**. It leads to malabsorption and steatorrhea. * **Predominant Species:** While the colon is 99% anaerobic, the duodenum contains mostly aerobic and facultative organisms (e.g., *Lactobacilli* and *Streptococci*). * **Blind Loop Syndrome:** Any anatomical stasis (e.g., diverticula or surgical loops) can cause the duodenal count to rise to colonic levels ($10^{10}$), causing Vitamin B12 deficiency.

Question 417: What is true of pepsinogen?

A. It is a condition in which it is safe to leave microscopic disease at the cut edges.
B. It shows a favorable response to radiotherapy.
C. It has a 5-year survival rate of about 12%. (Correct Answer)
D. Its rates are increased in patients with duodenal ulcer.

Explanation: This question appears to contain a common nomenclature error often seen in medical entrance exams, where **"Pepsinogen"** is used as a synonym for **Gastric Carcinoma (Stomach Cancer)**, specifically in the context of its biochemical markers and clinical outcomes. ### **Explanation of the Correct Option** **C. It has a 5-year survival rate of about 12%:** This is a classic statistical fact associated with advanced gastric adenocarcinoma. While early gastric cancer has a high survival rate, the overall 5-year survival rate for gastric cancer (often tested under the heading of pepsinogen/gastrin-related pathology) is historically cited around 10-15% (average 12%) due to late-stage presentation. ### **Analysis of Incorrect Options** * **A & B:** These options describe surgical and oncological principles that do not apply to gastric cancer. Leaving microscopic disease at the margins (R1 resection) is never "safe" in gastric cancer as it leads to high recurrence. Furthermore, gastric adenocarcinoma is generally considered **radioresistant**; surgery and chemotherapy are the primary treatment modalities. * **D. Its rates are increased in patients with duodenal ulcer:** This is physiologically incorrect. Patients with **Duodenal Ulcers (DU)** typically have high acid output and high pepsinogen levels, but they have a **decreased risk** of developing gastric cancer. Conversely, Gastric Ulcers and atrophic gastritis (associated with low pepsinogen I levels) are linked to a higher risk of malignancy. ### **NEET-PG High-Yield Pearls** * **Pepsinogen I vs. II:** A low Serum Pepsinogen I level and a low Pepsinogen I/II ratio are used as "serological biopsies" to screen for **atrophic gastritis**, a precursor to the intestinal type of gastric cancer. * **Most Common Site:** The pyloric antrum is the most common site for gastric cancer (50-60%). * **Virchow’s Node:** Left supraclavicular lymphadenopathy is a classic sign of metastatic gastric malignancy. * **Lauren Classification:** Divides gastric cancer into **Intestinal** (associated with H. pylori and environmental factors) and **Diffuse** (associated with signet ring cells and CDH1 mutations).

Question 418: In infants, defecation often follows a meal. What is the cause of colonic contractions in this situation?

A. Gastroileal reflex
B. Enterogastric reflex
C. Increased circulating levels of GCR
D. Gastrocolic reflex (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Gastrocolic reflex**. **Mechanism:** The gastrocolic reflex is a physiological reflex where distension of the stomach by food triggers mass movements in the colon. This reflex is mediated by the **parasympathetic nervous system** (vagus nerve) and gastrointestinal hormones like **gastrin and cholecystokinin (CCK)**. In infants, this reflex is particularly prominent because the inhibitory control from the higher centers of the brain (cortical control) is not yet developed, leading to the characteristic "feed and poop" pattern. **Analysis of Incorrect Options:** * **A. Gastroileal reflex:** This reflex occurs when food in the stomach triggers the relaxation of the **ileocecal valve**, allowing the contents of the small intestine to empty into the large intestine. It does not directly cause colonic mass movements. * **B. Enterogastric reflex:** This is an inhibitory reflex where distension or acidity in the duodenum inhibits gastric motility and secretions to slow down stomach emptying. It is the opposite of what occurs during defecation. * **C. Increased circulating levels of GCR:** GCR (Glucocorticoid Receptor) is unrelated to acute post-prandial colonic contractions. While hormones like Gastrin and CCK are involved, GCR plays no role in this reflex. **High-Yield Clinical Pearls for NEET-PG:** * **Mass Movements:** These are modified peristaltic waves that occur 1–3 times daily, typically after meals, to propel feces into the rectum. * **Neural Pathway:** The gastrocolic reflex is primarily mediated via the **pelvic splanchnic nerves** (parasympathetic). * **Clinical Correlation:** In adults, an overactive gastrocolic reflex is a hallmark of **Irritable Bowel Syndrome (IBS)**, leading to post-prandial urgency.

Question 419: All of the following are seen in persistent vomiting EXCEPT?

A. Hypokalemia
B. Decreased K+ in urine (Correct Answer)
C. Elevated pH of blood
D. Metabolic alkalosis

Explanation: In persistent vomiting, the body undergoes complex electrolyte and acid-base shifts. While it may seem counterintuitive, **urinary potassium levels actually increase**, making "Decreased K+ in urine" the correct exception. ### Why "Decreased K+ in urine" is the Correct Answer In persistent vomiting, there is a massive loss of HCl, leading to **Metabolic Alkalosis** (elevated blood pH). To compensate, the kidneys attempt to excrete excess bicarbonate ($HCO_3^-$). Because $HCO_3^-$ is a negatively charged ion, it must be excreted with a cation to maintain electrical neutrality. Since the body is trying to conserve Sodium ($Na^+$) due to dehydration (via Aldosterone), it excretes **Potassium ($K^+$)** instead. This phenomenon is known as **Paradoxical Kaliuresis**, leading to high levels of $K^+$ in the urine despite systemic hypokalemia. ### Why the other options are wrong: * **Hypokalemia (A):** Occurs due to three reasons: direct loss in vomitus, decreased oral intake, and primarily, the renal loss (kaliuresis) mentioned above. * **Elevated pH of blood (C) & Metabolic Alkalosis (D):** These are hallmarks of vomiting. Loss of gastric $H^+$ ions directly increases blood $HCO_3^-$ levels, raising the pH. ### NEET-PG High-Yield Pearls: * **Paradoxical Aciduria:** In severe, prolonged vomiting, the kidney eventually prioritizes $Na^+$ reabsorption over $H^+$ excretion to maintain blood volume. It begins excreting $H^+$ instead of $K^+$, leading to acidic urine despite systemic alkalosis. * **The "Vomiting Triad":** Hypokalemic, Hypochloremic, Metabolic Alkalosis. * **Aldosterone's Role:** Dehydration triggers the Renin-Angiotensin-Aldosterone System (RAAS). Aldosterone acts on the distal tubule to reabsorb $Na^+$ and excrete $K^+$ and $H^+$, further worsening the alkalosis and hypokalemia.

Question 420: Which of the following factors increases gastric motility?

A. Presence of fatty food in the stomach
B. Distension of the stomach (Correct Answer)
C. Presence of tryptophan in the stomach
D. Presence of acid in the stomach

Explanation: ### Explanation **Correct Answer: B. Distension of the stomach** **Why it is correct:** Gastric motility is primarily regulated by the volume of food within the stomach. When food enters, **distension** triggers stretch receptors in the gastric wall. This initiates **vagovagal reflexes** and local **myenteric reflexes**, which stimulate the release of **Gastrin**. Gastrin, along with parasympathetic stimulation, increases the force of antral contractions and promotes gastric emptying. Essentially, the stomach acts to move its contents forward once it is filled. **Why the other options are incorrect:** Options A, C, and D are all factors that trigger the **Enterogastric Reflex**. This reflex is inhibitory and originates from the **duodenum**, not the stomach, to ensure that the small intestine is not overwhelmed. * **A. Presence of fatty food:** Fats are the most potent inhibitors of gastric emptying. They trigger the release of **Cholecystokinin (CCK)**, which slows motility to allow more time for fat emulsification and digestion. * **C. Presence of tryptophan:** Amino acids (like tryptophan) in the duodenum stimulate the release of hormones like CCK and secretin, which inhibit gastric emptying. * **D. Presence of acid:** Low pH (< 3.5–4.0) in the duodenum triggers the release of **Secretin**, which inhibits gastric motility to protect the duodenal mucosa and allow neutralization by pancreatic bicarbonate. **High-Yield Clinical Pearls for NEET-PG:** * **Gastrin:** The only major GI hormone that *increases* gastric motility. * **CCK:** The most potent inhibitor of gastric emptying ("The Brake"). * **Migrating Motor Complex (MMC):** Occurs during fasting (inter-digestive state), mediated by **Motilin**, to clear the stomach of undigested debris. * **Vagotomy:** Often results in gastric stasis (delayed emptying) due to the loss of parasympathetic excitatory input.

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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Regulation of Food Intake

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