Digestive System — Set 5

Correct Answer: B. Vitamin K

• **Vitamin K** = Symbiotic bacteria such as Escherichia coli (E. coli) residing in the large intestine synthesise Vitamin K2 (menaquinone), which is then absorbed and used by the body for blood clotting and bone metabolism. • **Blood clotting** — Vitamin K is essential for activating clotting factors II, VII, IX, and X; a deficiency leads to excessive bleeding. • This is one reason prolonged antibiotic use can impair coagulation — antibiotics kill gut bacteria, reducing Vitamin K production. • 💡 Option A (Vitamin D) is wrong because Vitamin D is primarily synthesised in the skin upon exposure to sunlight (UV-B radiation); Option C (Vitamin C) is wrong because humans cannot synthesise Vitamin C and must obtain it from fruits and vegetables; Option D (Vitamin A) is wrong because Vitamin A is obtained from diet (liver, dairy, carrots) and is not made by gut bacteria.

Correct Answer: B. Bilirubin

• **Bilirubin** = Bilirubin is a yellow-orange pigment formed in the liver from the breakdown of haem (the iron-containing component of haemoglobin) when old red blood cells are destroyed; it is excreted in bile. • **Jaundice** — when bilirubin accumulates in the blood (due to liver disease, blocked bile ducts, or excessive red cell breakdown), it deposits in the skin and eyes, causing the characteristic yellow discolouration. • Bilirubin is also responsible for the brown colour of faeces (after gut bacteria convert it to stercobilin) and the yellow colour of urine (urobilin). • 💡 Option A (Melanin) is wrong because melanin is the skin and hair pigment responsible for colour and UV protection, not a bile component; Option C (Hemoglobin) is wrong because haemoglobin is the oxygen-carrying protein in red blood cells and is the precursor to bilirubin, not the pigment itself; Option D (Chlorophyll) is wrong because chlorophyll is a plant pigment used in photosynthesis and is not found in the human body.

Correct Answer: A. Ileocaecal valve

• **Ileocaecal valve** = This one-way valve sits at the junction of the ileum (last part of the small intestine) and the caecum (first part of the large intestine/colon), allowing digested material to pass from the small to the large intestine while preventing backflow. • **Prevention of contamination** — it stops bacteria-laden large-intestinal contents from re-entering the small intestine, protecting the nutrient-absorbing region from microbial colonisation. • Its controlled opening ensures the slow, regulated transfer of chyme, giving the large intestine time to absorb water. • 💡 Option B (Pyloric sphincter) is wrong because it controls the flow of food from the stomach into the duodenum, not from the ileum to the colon; Option C (Uvula) is wrong because the uvula is the fleshy projection at the back of the soft palate in the mouth/throat; Option D (Cardiac sphincter) is wrong because the cardiac (lower esophageal) sphincter is at the junction of the esophagus and stomach.

Correct Answer: B. Liver

• **Liver** = Called the body's chemical factory, the liver receives nutrient-rich blood directly from the digestive system via the hepatic portal vein and processes it — storing glycogen, synthesising proteins, metabolising fats, and detoxifying harmful substances. • **Detoxification** — the liver converts ammonia (from protein metabolism) to urea, and neutralises drugs, alcohol, and other toxins before they reach general circulation. • It also stores vitamins A, D, E, K, and B12, and produces clotting factors and bile. • 💡 Option A (Heart) is wrong because the heart is a pumping organ responsible for circulation, not chemical processing of nutrients; Option C (Kidney) is wrong because kidneys filter blood and excrete waste as urine but do not process absorbed nutrients; Option D (Lungs) is wrong because lungs facilitate gas exchange (O₂/CO₂) and have no role in nutrient metabolism.

Correct Answer: B. 8

• **8** = There are 4 incisors in the upper jaw and 4 in the lower jaw, giving a total of 8 incisors in the permanent dentition. They are chisel-shaped and located at the front of the mouth. • **Function** — incisors are used for biting and cutting food into smaller pieces before chewing begins; their flat, sharp edges make them ideal for this purpose. • An adult has 32 permanent teeth in total: 8 incisors, 4 canines, 8 premolars, and 12 molars (including 4 wisdom teeth). • 💡 Option A (4) is wrong because 4 would represent only the upper or lower incisors, not the full set; Option C (12) is wrong because 12 is the number of molars in permanent dentition; Option D (16) is wrong because there is no tooth type with 16 members in the human dental formula.

Correct Answer: D. Pancreatic Amylase

• **Pancreatic Amylase** = Secreted by the pancreas into the duodenum, pancreatic amylase continues the carbohydrate digestion that salivary amylase began in the mouth, hydrolyzing the remaining starch chains into maltose and other short oligosaccharides. • **Alkaline environment** — pancreatic amylase works optimally in the alkaline pH (around 7–8) of the duodenum, maintained by bicarbonate ions secreted by the pancreas. • The maltose produced is then further broken down into glucose by the brush-border enzyme maltase. • 💡 Option A (Lactase) is wrong because lactase specifically breaks down lactose (milk sugar), not starch; Option B (Pepsin) is wrong because pepsin is a protease that digests proteins in the acidic stomach, not a carbohydrase; Option C (Bile) is wrong because bile is not an enzyme — it emulsifies fats but does not chemically digest starch.

Correct Answer: A. Rectum

• **Rectum** = The rectum is the final 15–20 cm straight section of the large intestine that connects the sigmoid colon to the anal canal; it acts as a temporary storage chamber for faeces before voluntary defecation. • **Defecation reflex** — as the rectum fills and its walls stretch, nerve signals trigger the urge to defecate; the internal anal sphincter relaxes involuntarily while the external sphincter is under voluntary control. • The rectum does not absorb significant nutrients but does absorb some water and electrolytes. • 💡 Option B (Colon) is wrong because the colon is the main section of the large intestine that absorbs water and compacts waste, but does not store faeces immediately before egestion; Option C (Anus) is wrong because the anus is the external opening through which faeces exit, not a storage organ; Option D (Caecum) is wrong because the caecum is the very beginning of the large intestine.

Correct Answer: D. Cholecystokinin

• **Cholecystokinin** = CCK is a hormone produced by I-cells in the mucosa of the duodenum and jejunum in response to the presence of fats and proteins in the partially digested food; it triggers contraction of the gallbladder and relaxation of the sphincter of Oddi, releasing bile into the duodenum. • **Dual action** — CCK also stimulates the pancreas to secrete digestive enzymes (pancreatic juice), making it a key co-ordinator of fat and protein digestion. • The bile released emulsifies fat globules into tiny droplets, dramatically increasing the surface area for lipase action. • 💡 Option A (Thyroxine) is wrong because thyroxine is a thyroid hormone that regulates metabolic rate and is unrelated to bile secretion; Option B (Insulin) is wrong because insulin is secreted by the pancreas to lower blood glucose and plays no role in bile release; Option C (Glucagon) is wrong because glucagon raises blood glucose by stimulating glycogen breakdown in the liver and does not trigger bile secretion.

Correct Answer: A. Killing bacteria

• **Killing bacteria** = Lysozyme is an antimicrobial enzyme present in saliva that attacks the peptidoglycan cell walls of bacteria, causing them to lyse (burst), thereby acting as a first line of immune defence in the oral cavity. • **Broad protection** — it is effective primarily against gram-positive bacteria and works alongside other salivary antimicrobials like immunoglobulin A (IgA) and lactoferrin to keep the mouth relatively pathogen-free. • Lysozyme is also found in tears, mucus, and breast milk, showing its wide role in innate immunity. • 💡 Option B (Breaking down proteins) is wrong because salivary amylase is the main digestive enzyme in saliva and it targets starch, not proteins; Option C (Absorption of water) is wrong because water absorption occurs primarily in the large intestine, not the mouth; Option D (Digestion of fats) is wrong because fat digestion begins in the small intestine via bile and lipase, not in the saliva.

Correct Answer: A. Third molars

• **Third molars** = Wisdom teeth are the third and final set of molars located at the very back of each quadrant of the jaw; they are called 'wisdom teeth' because they erupt much later than other teeth, typically between ages 17 and 25, a time associated with maturity. • **4 wisdom teeth** — one in each corner of the mouth (upper-left, upper-right, lower-left, lower-right), though many people have fewer due to evolutionary reduction or impaction. • Impacted wisdom teeth (those that cannot fully emerge due to lack of space) often require surgical removal. • 💡 Option B (Second molars) is wrong because second molars erupt around age 11–13 and are not called wisdom teeth; Option C (Premolars) is wrong because premolars erupt around age 10–12 and are bicuspid teeth located before the molars; Option D (First molars) is wrong because first molars are the first permanent back teeth to erupt, around age 6, and are sometimes called 'six-year molars'.