Digestive System — Set 3

Correct Answer: C. 2123/2123

• **2123/2123** = this formula means 2 incisors, 1 canine, 2 premolars, and 3 molars per quadrant (upper and lower, left and right), giving 8 teeth per quadrant × 4 quadrants = 32 permanent teeth in total in an adult human. • **Dental formula significance** — the formula 2123 is written for one half of the jaw (one quadrant); reading it: incisors cut food, canines tear, premolars crush, and molars grind, reflecting the different functional shapes. • 💡 Option A (2102/2102) is wrong because it shows 0 premolars and only 2 molars per quadrant, which does not match the human dentition; Option B (2133/2133) is wrong because it incorrectly lists 3 premolars per quadrant; Option D (2023/2023) is wrong because it omits canines entirely, which humans clearly possess.

Correct Answer: D. Hepatic Lobule

• **Hepatic Lobule** = a hexagonal structural unit (~1–2 mm diameter) consisting of plates of hepatocytes (liver cells) arranged around a central vein, with portal triads at the corners; it is the repeating functional module responsible for all liver metabolic activities. • **Portal triad** — each corner of the hepatic lobule contains a branch of the portal vein, hepatic artery, and bile ductule; blood flows inward from the portal triad to the central vein while bile flows outward, enabling simultaneous blood processing and bile production. • 💡 Option A (Hepatocyte) is wrong because a hepatocyte is a single liver cell — the building block of the lobule, not the structural unit itself; Option B (Nephron) is wrong because nephron is the functional unit of the kidney; Option C (Alveolus) is wrong because alveolus is the functional unit of the lung.

Correct Answer: A. Intestinal juice

• **Intestinal juice (Succus Entericus)** = secreted by the crypts of Lieberkühn in the small intestine lining, it contains brush-border enzymes including maltase, sucrase, lactase, and peptidases that complete the final steps of carbohydrate and protein digestion. • **Alkaline pH** — Succus Entericus has a pH of about 7.8–8.0 and contains mucus that protects the intestinal lining from its own digestive enzymes and the remaining acidity of chyme. • 💡 Option B (Gastric juice) is wrong because gastric juice is secreted by stomach glands and contains HCl and pepsin; Option C (Pancreatic juice) is wrong because pancreatic juice contains trypsin, lipase, and amylase and comes from the pancreas; Option D (Bile juice) is wrong because bile is produced by the liver and stored in the gallbladder, not secreted by the intestinal lining.

Correct Answer: D. Liver

• **Liver** = Kupffer cells are resident macrophages lining the hepatic sinusoids (small blood channels between hepatocyte plates); they phagocytose bacteria, worn-out red blood cells, cellular debris, and foreign particles from the blood as it passes through the liver from the portal vein. • **Immune sentinel** — Kupffer cells are the largest population of tissue-resident macrophages in the body; they also break down the hemoglobin of old RBCs into bilirubin, which the liver then secretes into bile. • 💡 Option A (Kidney) is wrong because the kidney contains mesangial cells and tubular cells for filtration, not Kupffer cells; Option B (Lungs) is wrong because the lungs contain alveolar macrophages for air-space immunity; Option C (Spleen) is wrong because the spleen does filter old RBCs but through a different mechanism involving macrophages of the red pulp, not Kupffer cells.

Correct Answer: D. Pulp Cavity

• **Pulp Cavity** = the innermost, soft-tissue core of the tooth, filled with pulp — a connective tissue containing blood vessels (which nourish the tooth), lymphatic vessels, and sensory nerve fibers (which transmit pain when the tooth is damaged or infected). • **Root canal treatment** — when the pulp becomes infected, dentists perform root canal therapy by removing the pulp tissue and sealing the cavity; the tooth can survive without pulp but loses its ability to sense temperature and pain. • 💡 Option A (Cementum) is wrong because cementum is the thin calcified layer covering the tooth root that anchors the periodontal ligament; Option B (Dentine) is wrong because dentine is the calcified tissue surrounding the pulp, which conducts sensation but does not itself contain major blood vessels; Option C (Enamel) is wrong because enamel is avascular and acellular with no blood supply or nerves.

Correct Answer: B. Water absorption

• **Water absorption** = the colon receives about 1–1.5 litres of fluid chyme daily from the small intestine and absorbs up to 90% of this water along with electrolytes like Na⁺ and Cl⁻, compacting the material into solid feces. • **Four segments** — the colon is divided into the ascending, transverse, descending, and sigmoid colon; bacterial fermentation of undigested fibres in all segments produces short-chain fatty acids and gases as by-products. • 💡 Option A (Protein digestion) is wrong because protein digestion is completed in the small intestine by trypsin, chymotrypsin, and peptidases; Option C (Fat absorption) is wrong because fat absorption occurs in the small intestine (jejunum) via lacteals; Option D (Storage of feces) is wrong because feces storage is the function of the rectum, not the colon.

Correct Answer: C. Rennin

• **Rennin (Chymosin)** = secreted by chief cells in the gastric glands of infants, rennin cleaves κ-casein (a stabilizing protein in milk), converting it to paracasein, which then forms a curd (clot) in the presence of calcium ions, slowing down the movement of milk through the stomach for better digestion. • **Adult disappearance** — rennin activity decreases sharply after infancy as pepsin takes over protein digestion; commercially, rennin is extracted from calf stomachs and used in cheese-making to curdle milk. • 💡 Option A (Pepsin) is wrong because while pepsin also digests proteins in the stomach, it is the adult enzyme and does not specifically act on casein the way rennin does; Option B (Amylase) is wrong because amylase digests starch/carbohydrates, not milk protein; Option D (Trypsin) is wrong because trypsin is a pancreatic protease acting in the small intestine, not in the infant stomach.

Correct Answer: D. Duodenum

• **Duodenum** = Brunner's glands are compound tubular mucous glands located exclusively in the submucosa of the duodenum (especially the proximal portion); they secrete highly alkaline (pH 8–9) mucus rich in bicarbonate to neutralize the extremely acidic chyme arriving from the stomach. • **Protective role** — their secretion creates a protective alkaline layer over the duodenal epithelium, shielding it from peptic ulceration; disorders of Brunner's glands can predispose to duodenal ulcers. • 💡 Option A (Ileum) is wrong because the ileum contains Peyer's patches (lymphoid tissue) but not Brunner's glands; Option B (Colon) is wrong because the colon contains simple tubular glands (crypts) secreting mucus, not the specialized Brunner's glands; Option C (Stomach) is wrong because the stomach has gastric glands secreting HCl, pepsinogen, and mucus, but not Brunner's glands.

Correct Answer: C. Vitamin A

• **Vitamin A** = a fat-soluble vitamin (along with D, E, and K, remembered as ADEK) that requires the presence of dietary fat and bile salts for emulsification before it can be packaged into chylomicrons and absorbed via the lacteals (lymphatic system) of the small intestine. • **Night blindness link** — Vitamin A (retinol) is a precursor of retinal, a component of the visual pigment rhodopsin; its deficiency leads to night blindness and xerophthalmia, making it one of the most clinically important fat-soluble vitamins. • 💡 Option A (Vitamin C) is wrong because Vitamin C (ascorbic acid) is water-soluble and absorbed directly into the bloodstream without requiring fat or bile; Option B (Vitamin B12) is wrong because it is water-soluble and requires intrinsic factor for absorption in the ileum, not dietary fat; Option D (Vitamin B6) is wrong because it is a water-soluble B-vitamin absorbed in the jejunum without fat dependence.

Correct Answer: B. Pancreas

• **Pancreas** = the Islets of Langerhans are scattered clusters of endocrine cells embedded within the exocrine tissue of the pancreas; they comprise about 1–2% of the pancreatic mass and contain alpha (α) cells secreting glucagon and beta (β) cells secreting insulin, directly into the bloodstream. • **Blood sugar regulation** — insulin (from β cells) lowers blood glucose after meals, while glucagon (from α cells) raises it during fasting; the balance between these two hormones maintains blood glucose homeostasis, and destruction of β cells leads to Type 1 diabetes. • 💡 Option A (Stomach) is wrong because the stomach secretes gastrin and ghrelin but does not contain the Islets of Langerhans; Option C (Liver) is wrong because the liver processes and stores glucose but does not produce insulin or house these islets; Option D (Gallbladder) is wrong because the gallbladder only stores bile and has no endocrine function.