Digestive System — Set 4

Correct Answer: C. Canines

• **Canines** = Sharp, pointed teeth located at the corners of the mouth, specifically designed to pierce and tear tough foods like meat. • **4 canines total** — one in each quadrant of the mouth (upper-left, upper-right, lower-left, lower-right). • They are the longest teeth in the jaw and have a single, deep root. • 💡 Option A (Incisors) is wrong because incisors are chisel-shaped and used for biting/cutting, not tearing; Option B (Molars) is wrong because molars are flat and used for grinding food; Option D (Premolars) is wrong because premolars do some crushing but are not the dedicated tearing teeth.

Correct Answer: B. Slightly acidic (around 6.8)

• **Slightly acidic (around 6.8)** = Human saliva normally ranges between pH 6.2–7.6, with 6.8 being the optimal range for salivary amylase to begin starch digestion in the mouth. • **Salivary amylase** — this enzyme works best in a slightly acidic to neutral environment and becomes inactive in the highly acidic stomach. • Saliva also contains lysozyme (antibacterial) and mucin (lubricant), making it more than just a digestive fluid. • 💡 Option A (Acidic ~2.0) is wrong because pH 2.0 is the pH of gastric juice in the stomach, not saliva; Option C (Neutral 7.0) is wrong because saliva is slightly below neutral; Option D (Highly alkaline ~10.0) is wrong because no digestive secretion in the mouth is that alkaline.

Correct Answer: B. Liver

• **Liver** = The liver is the largest gland in the human body, weighing about 1.5 kg in adults, and performs over 500 essential functions including bile production, detoxification, and glycogen storage. • **Bile production** — the liver produces about 500–1000 mL of bile per day, which is stored in the gallbladder and released to emulsify fats. • It also synthesises plasma proteins, regulates blood glucose, and converts ammonia to urea. • 💡 Option A (Pancreas) is wrong because the pancreas is a mixed gland but much smaller than the liver; Option C (Thyroid) is wrong because the thyroid is an endocrine gland and far smaller; Option D (Adrenal) is wrong because adrenal glands sit atop kidneys and are very small.

Correct Answer: A. Acidic

• **Acidic** = Pepsin is a protease secreted as the inactive precursor pepsinogen by chief cells of the stomach; it is activated and works optimally at pH 1.5–2.5, which is maintained by HCl secreted by parietal cells. • **Pepsinogen → Pepsin** — HCl in the stomach triggers this conversion, allowing protein chains to be cleaved into smaller peptides. • If the stomach becomes alkaline (e.g., after antacids), pepsin is inactivated and protein digestion stalls until pancreatic proteases take over in the duodenum. • 💡 Option B (Alkaline) is wrong because pepsin is denatured in alkaline conditions; Option C (Saline) is wrong because saline refers to salt solution, not an enzyme-activating pH; Option D (Neutral) is wrong because pepsin needs a much lower pH than 7.0 to function.

Correct Answer: D. Esophagus

• **Esophagus** = Commonly called the food pipe, it is a muscular tube about 25 cm long that connects the pharynx to the stomach, propelling food downward through rhythmic contractions called peristalsis. • **Peristalsis** — these wave-like muscular movements push a bolus of food from the throat to the stomach in about 8–10 seconds. • The lower end of the esophagus has the lower esophageal sphincter (cardiac sphincter) which prevents stomach acid from flowing back up. • 💡 Option A (Larynx) is wrong because the larynx is the voice box and is part of the respiratory tract, not the digestive tract; Option B (Trachea) is wrong because the trachea is the windpipe that carries air to the lungs; Option C (Pharynx) is wrong because the pharynx is a shared passage for food and air but is not the food pipe itself.

Correct Answer: B. Lower esophageal sphincter

• **Lower esophageal sphincter** = Also called the cardiac sphincter, it is a ring of smooth muscle at the junction of the esophagus and stomach that stays closed to prevent the acidic stomach contents from refluxing into the esophagus. • **GERD** — when this sphincter weakens or relaxes inappropriately, gastroesophageal reflux disease (acid reflux/heartburn) occurs. • It opens only when a food bolus arrives from above, then quickly closes again. • 💡 Option A (Anal sphincter) is wrong because it controls defecation at the very end of the digestive tract; Option C (Ileocaecal valve) is wrong because it sits between the small intestine and large intestine; Option D (Pyloric sphincter) is wrong because it controls the exit of food from the stomach into the duodenum, not the esophagus-stomach junction.

Correct Answer: B. Maltase

• **Maltase** = Maltase is a brush-border enzyme found in the intestinal microvilli that specifically hydrolyses maltose (a disaccharide) into two molecules of glucose, completing carbohydrate digestion. • **Brush-border enzymes** — maltase, lactase, and sucrase are all located on the surface of intestinal epithelial cells, acting on disaccharides right at the absorption site. • The resulting glucose molecules are then absorbed into the bloodstream via active transport. • 💡 Option A (Lactase) is wrong because lactase breaks down lactose (milk sugar) into glucose and galactose, not maltose; Option C (Sucrase) is wrong because sucrase breaks down sucrose (table sugar) into glucose and fructose; Option D (Amylase) is wrong because amylase breaks starch into maltose but cannot itself break maltose into glucose.

Correct Answer: D. Caecum

• **Caecum** = The caecum is a pouch-like structure that forms the beginning of the large intestine, located in the lower right abdomen; it receives incompletely digested material from the ileum through the ileocaecal valve. • **Appendix** — the vermiform appendix is a finger-like projection attached to the posteromedial wall of the caecum; its exact function is debated but it may house beneficial gut bacteria. • Inflammation of the appendix is called appendicitis, which is a medical emergency. • 💡 Option A (Colon) is wrong because the colon is the main body of the large intestine that comes after the caecum; Option B (Anus) is wrong because the anus is the final opening for excretion; Option C (Rectum) is wrong because the rectum is the last section before the anus where feces are stored.

Correct Answer: B. Mucus

• **Mucus** = Mucus is secreted by goblet cells and mucous neck cells in the stomach lining, forming a thick gel-like layer (about 0.5 mm thick) that acts as a physical barrier between the corrosive HCl and the stomach wall. • **Bicarbonate ions** — the mucus layer also traps bicarbonate (secreted by surface cells), which neutralises any acid that penetrates close to the epithelium, keeping the surface pH near 7. • When this protective layer is disrupted (e.g., by H. pylori bacteria or NSAIDs), peptic ulcers form. • 💡 Option A (Gastrin) is wrong because gastrin is a hormone that actually stimulates HCl secretion rather than protecting the wall; Option C (Pepsin) is wrong because pepsin is a protease that would damage tissue if not for the mucus barrier; Option D (Bile) is wrong because bile is produced in the liver and is not a stomach protectant.

Correct Answer: D. Breaking down milk sugar

• **Breaking down milk sugar** = Lactase is a brush-border enzyme produced in the small intestine that hydrolyses lactose (the disaccharide sugar in milk) into glucose and galactose, which can then be absorbed. • **Lactose intolerance** — individuals who lack sufficient lactase cannot digest lactose; it passes undigested to the colon where bacteria ferment it, causing bloating, gas, and diarrhoea. • Lactase activity is highest in infancy and decreases with age in most of the world's population. • 💡 Option A (Emulsifying fats) is wrong because fats are emulsified by bile salts, not an enzyme; Option B (Digesting starch) is wrong because starch is broken down by amylase; Option C (Digesting proteins) is wrong because proteins are digested by proteases like pepsin and trypsin.