MPSC 2024 Prelims Paper-I · Question 63 of 100

Correct — option (1), the pairing (a) – (iv), (b) – (iii), (c) – (ii), (d) – (i). List no. 1 runs down the forelimb and List no. 2 down the hindlimb, and the match asked for is the serial homology between them: the two limbs of a tetrapod are built on the same plan, so each element of the front limb has a counterpart in the same position in the hind limb. Taking the left column in order, the shoulder joint (a) is the joint between the pectoral girdle and the first limb bone, and its counterpart is the hip joint (iv), the joint between the pelvic girdle and the first limb bone — hence (a) – (iv). The humerus (b) is the single long bone of the upper arm, and the single long bone of the thigh is the femur (iii) — hence (b) – (iii). The elbow (c) is the hinge between the first and the second segment of the forelimb, and the knee (ii) is the corresponding hinge of the hindlimb — hence (c) – (ii). That leaves the radio-ulna (d), the second-segment bone of the forelimb, matched with the tibio-fibula (i), the second-segment bone of the hindlimb. Notice that the two lists are printed side by side only as layout; the pairing has to be argued, and here it can be argued twice over. First, a joint can only be homologous to a joint and a bone to a bone: the left column reads joint, bone, joint, bone, and the right column reads bone, joint, bone, joint, so the only pairings that can even be considered are (a) with (ii) or (iv), (b) with (i) or (iii), and so on. That single parity rule destroys options (2) and (4) immediately and option (3) at its second pairing. Second, the order along the limb settles what is left: girdle joint, first long bone, middle hinge, second-segment bone, in that sequence in both limbs. The paired names radio-ulna and tibio-fibula are worth noting in themselves: they are the fused forearm and shank bones described in the frog, whereas in the human the same elements remain separate as radius and ulna, tibia and fibula.

• (2)(a) – (iii), (b) – (ii), (c) – (iv), (d) – (i) — This option pairs the shoulder joint with the femur, a joint with a bone, and then the humerus with the knee, a bone with a joint — both impossible as homologies. Only its last pairing, radio-ulna with tibio-fibula, is right, and one correct pairing at the end of a wrong sequence is exactly how a match option is built to look plausible. The first two pairings can be rejected without any anatomy beyond noticing that a joint is not a bone, which is why the parity check is worth running before anything else in a matching question.
• (3)(a) – (ii), (b) – (iv), (c) – (i), (d) – (iii) — This option starts correctly in kind — the shoulder joint is matched with the knee, a joint with a joint — but it is the wrong joint, since the shoulder is the girdle joint and its counterpart is the hip, not the middle hinge of the limb. It then breaks down completely: the humerus is paired with the hip joint, the elbow with the tibio-fibula and the radio-ulna with the femur, three pairings that each cross a bone with a joint or place an element at the wrong position along the limb. The option is aimed at the candidate who knows the two limbs are homologous but has not fixed the order of elements from the girdle outwards.
• (4)(a) – (i), (b) – (ii), (c) – (iv), (d) – (iii) — This is the option a candidate drifts to by pairing the two lists down the page without reading what is in them — though it is only the nearest thing to that, not the thing itself: it takes the first two rows in order, (a) with (i) and (b) with (ii), and then crosses the last two, (c) with (iv) and (d) with (iii). No option in this question offers the pure straight-down sequence, which is part of why the mechanical approach feels plausible here and still fails. Every one of its four pairings is wrong: the shoulder joint is matched with the tibio-fibula, the humerus with the knee, the elbow with the hip joint and the radio-ulna with the femur, so joints and bones are crossed throughout. The Commission prints the right-hand column deliberately out of order for this reason — adjacency on the page is layout, never an answer, and an option that simply follows the printed order is the standard trap in this format.

The forelimb and the hindlimb of a tetrapod are built on one plan, the pentadactyl or five-fingered limb, and the correspondence between them is called serial homology — the same structure repeated in a different position of the same body. The plan runs: a girdle, then a single long bone, then a hinge joint, then a pair of bones, then the small bones of the wrist or ankle, then the digits. In the forelimb this is the pectoral girdle, the humerus, the elbow, the radius and ulna, the carpals, the metacarpals and the phalanges; in the hindlimb it is the pelvic girdle, the femur, the knee, the tibia and fibula, the tarsals, the metatarsals and the phalanges. The joint at which the limb meets its girdle is the shoulder in front and the hip behind, and both are ball-and-socket joints, while the elbow and the knee are hinge joints permitting movement in one plane. In the frog, the animal on which Indian zoology courses teach limb anatomy, the two bones of the second segment are fused in each limb, giving the single radio-ulna in the forelimb and the single tibio-fibula in the hindlimb, which is why those two names are printed here rather than the four separate human bones. The concept matters beyond the naming: homologous organs — the same underlying structure serving different functions — are the classical anatomical evidence for common ancestry, as against analogous organs, which serve the same function from different structures.

MPSC's match-the-columns questions in biology are usually answerable by a structural rule rather than by recalling every item, and this question is the clearest example of that. Two rules do the work here: like matches like, so a joint pairs only with a joint, and position along the limb is preserved, so the girdle joint pairs with the girdle joint and the second-segment bone with the second-segment bone. Running the first rule across all four options takes a few seconds and usually leaves a single survivor. The wider habit the Commission is testing is the discipline of treating the two printed columns as unpaired until the candidate has paired them: they are set side by side because they have to be matched, and any option that simply reproduces the printed order should be treated with suspicion rather than confidence.

• The forelimb and hindlimb of tetrapods are serially homologous: girdle, single long bone, hinge joint, paired bones, wrist or ankle bones, digits.
• The shoulder joint of the forelimb corresponds to the hip joint of the hindlimb; both are ball-and-socket joints between the limb and its girdle.
• The humerus of the upper arm corresponds to the femur of the thigh, each being the single long bone of the first limb segment.
• The elbow corresponds to the knee; both are hinge joints between the first and second segments of the limb.
• In the frog the second-segment bones are fused, giving the radio-ulna in the forelimb and the tibio-fibula in the hindlimb, while in the human they remain separate as radius and ulna, tibia and fibula.

• Pairing the two printed columns straight down the page; the right-hand column is deliberately printed out of order
• Matching a joint with a bone — the parity check between the two columns is the fastest way to eliminate options here
• Confusing the girdle joint with the middle hinge, so pairing the shoulder with the knee instead of with the hip
• Expecting the human names radius, ulna, tibia and fibula; this question prints the fused frog bones radio-ulna and tibio-fibula

Human and comparative anatomy appears in MPSC papers both as direct recall — how many bones, which bone is where, which joint is of what type — and as match-the-columns items of exactly this kind, pairing bones with regions, joints with types, or forelimb elements with their hindlimb counterparts. The columns are always printed side by side and never in the answer order. The efficient method is to look first for a structural rule that partitions the two columns, such as bone against joint or proximal against distal, apply it to every option at once, and only then check the survivors item by item.

No directly related past PYQ was found.

• practice — not a real PYQ

  In the human limb skeleton, the bone of the hindlimb that corresponds serially to the humerus of the forelimb is :

  • (a)Tibia
  • (b)Femur
  • (c)Fibula
  • (d)Patella

  Answer(b) Femur — the humerus is the single long bone of the first segment of the forelimb, and the femur occupies exactly that position in the hindlimb. The tibia and fibula correspond to the radius and ulna of the second segment, and the patella is the sesamoid bone of the knee, which has no counterpart at the elbow.

• practice — not a real PYQ

  The shoulder joint and the hip joint are both examples of which type of joint ?

  • (a)Hinge joint
  • (b)Pivot joint
  • (c)Ball-and-socket joint
  • (d)Gliding joint

  Answer(c) Ball-and-socket joint — each is formed where the rounded head of a limb bone sits in a socket of its girdle, allowing movement in all planes. The elbow and the knee, by contrast, are hinge joints permitting movement in one plane only.