The inner lining of the human small intestine has numerous finger-like projections called:

Correct — B, Villi. The stem is the textbook sentence with the last word removed. Villi are microscopic finger-shaped folds of the intestinal lining, roughly half a millimetre to one millimetre tall, and their whole purpose is arithmetic: absorption happens across a surface, so the more surface the tube can pack into the same length, the more of the digested food it can take up. The small intestine multiplies its area three times over — circular folds of the wall, villi standing on those folds, and microvilli forming a brush border on each villus cell — which together turn a tube of a few square metres of bare wall into an absorptive surface of the order of a couple of hundred square metres. Each villus is richly supplied with blood capillaries that carry away glucose and amino acids, and it also contains a lymph vessel, the lacteal, which takes up the products of fat digestion. That is why the small intestine, and not the stomach or the large intestine, is where almost all nutrient absorption happens.

• (a)Sphincter — A ring of muscle that closes a passage, not a projection of the lining. The same NCERT passage names the anal sphincter, which regulates the exit of waste; the pyloric sphincter guards the stomach outlet.
• (c)Enzymes — Chemicals, not structures. Enzymes such as trypsin, lipase and amylase act inside the small intestine to break food down, but they are secreted into the cavity — they are not what the wall is folded into.
• (d)Plaques — Not part of normal intestinal anatomy in this sense. In medicine a plaque is a deposit — the fatty plaque of atherosclerosis in an artery, or dental plaque on a tooth — that is, something laid down abnormally rather than a healthy fold of tissue.

The small intestine is the site of complete digestion and of nearly all absorption. Bile from the liver emulsifies fat, pancreatic juice supplies trypsin, lipase and amylase, and the intestinal glands finish the job, so that proteins become amino acids, carbohydrates become glucose and fats become fatty acids and glycerol. The absorbing surface is enlarged by villi, each carrying a capillary network and a lacteal, and the villus cells themselves carry microvilli. What is not absorbed passes to the large intestine, whose wall absorbs more water before the residue leaves through the anus.

Every wrong option here belongs to a different category of thing, which is the quickest route to the answer: a muscle ring, a class of chemicals, and a pathological deposit against a fold of the lining. If the categories do not come to mind, use the design principle instead. Any organ whose job is exchange across a surface solves the same problem the same way — villi in the intestine, alveoli in the lungs, root hairs in a plant, the convolutions of the nephron. Recognising that family is worth more marks than the single word. Note also the sequence the same passage gives, because it is asked as often as the villi are: emulsification of fat by bile, enzymatic breakdown by pancreatic and intestinal juice, absorption through the villi, then water absorption in the large intestine.

• Villi are finger-like projections of the inner lining of the small intestine that increase the surface area available for absorption.
• Each villus is richly supplied with blood vessels that carry absorbed glucose and amino acids to the cells of the body.
• The small intestine is the longest part of the human alimentary canal, which is what allows food to be held long enough for complete digestion and absorption.
• Bile from the liver emulsifies fats and makes the medium alkaline; pancreatic juice supplies trypsin for proteins and lipase for emulsified fats.
• The large intestine absorbs water from the unabsorbed material, and the anal sphincter — a muscle ring, not a projection — regulates the exit of waste.

• Confusing villi with microvilli — the villi are visible folds of the wall, the microvilli a brush border on each cell of a villus.
• Assuming absorption happens mainly in the stomach; the stomach digests, but nearly all absorption is in the small intestine.
• Reading 'sphincter' as any structure of the gut wall rather than specifically a ring of muscle guarding a passage.

Usually as this one-line recall item, or as 'which part of the alimentary canal is the longest', or by asking what increases the absorptive surface of the small intestine.

• practice — not a real PYQ

  The lacteal present inside a villus of the small intestine is a vessel of which system?

  • (a)Blood circulatory system
  • (b)Lymphatic system
  • (c)Nervous system
  • (d)Excretory system

  Answer(b) Lymphatic system — the lacteal is a lymph vessel, and it absorbs the products of fat digestion, while the capillaries of the villus take up glucose and amino acids.

• practice — not a real PYQ

  Which one of the following is the principal function of the large intestine in human beings?

  • (a)Absorption of amino acids
  • (b)Secretion of bile
  • (c)Absorption of water from the unabsorbed food
  • (d)Emulsification of fats

  Answer(c) Absorption of water from the unabsorbed food — nutrient absorption is finished in the small intestine, and the wall of the large intestine then recovers water before the residue is expelled.