Match the following to express general resemblance between bones and girdle fore limbs and hind limbs.
List no. - 1
(a) Shoulder joint
(b) Humerus
(c) Elbow
(d) Radio-ulna
List no. - 2
(i) Tibio-fibul

Question

Match the following to express general resemblance between bones and girdle fore limbs and hind limbs.
List no. - 1
(a) Shoulder joint
(b) Humerus
(c) Elbow
(d) Radio-ulna
List no. - 2
(i) Tibio-fibula
(ii) Knee
(iii) Femur
(iv) Hip joint
Answer options :

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (b) 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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2024 Q63 — Science & Technology PYQ with Answer

State PCS 2024 Q63 — Science & Technology PYQ with Answer