The Ishihara test is related with

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Correct — A, Colour blindness. The Ishihara test is a colour vision test for detecting red-green colour deficiencies, named after Shinobu Ishihara, a professor at the University of Tokyo, who first published it in 1917. It works through pseudo-isochromatic plates: each plate is a solid circle filled with coloured dots of deliberately varied size and shade, and hidden inside that apparent randomness is a number or a shape picked out in a confusable hue. Someone with normal colour vision reads one figure; someone with a red-green deficiency reads a different one, or none at all. The best-known plate is the one on which normal vision reads 74 and red-green deficient vision reads 21. The full set runs to thirty-eight plates, though shorter versions of ten, fourteen or twenty-four are common and a severe deficiency usually shows within the first few. The plates are of several deliberate kinds — demonstration plates visible to everyone, transformation plates that show different figures to different viewers, vanishing plates readable only by normal vision, hidden-digit plates readable only by the colour-deficient, diagnostic plates that distinguish protanopia from deuteranopia, and tracing plates on which the subject follows a line rather than reads a number. That variety of design is what makes it a screening tool rather than a party trick. Each of the other three options names a real clinical measurement, and each has its own instrument.

Why the other options are wrong — (b) Haemoglobin concentration is measured on a blood sample, not by looking at a chart — historically by the Sahli haemoglobinometer, which matched a treated blood sample against a colour standard, and now by automated cell counters or point-of-care photometers. The colour-matching step in the old Sahli method is the only thing this option shares with Ishihara, and it is a coincidence of technique, not of purpose.  (c) Intelligence is assessed by standardised psychometric batteries — the Stanford-Binet scales, the Wechsler scales, or Raven's Progressive Matrices, which are themselves pattern-based and therefore superficially resemble a plate test. But Raven's matrices test reasoning about form and sequence and are deliberately designed to be culture-fair and colour-independent, which is the opposite of what Ishihara plates do.  (d) Bone mineral density is measured by dual-energy X-ray absorptiometry, the DEXA scan, which passes two X-ray energies through the skeleton and computes density from the difference in absorption; the result is reported as a T-score and is used to diagnose osteoporosis. It is an imaging investigation and shares nothing with a printed colour plate.

Concept: Human colour vision depends on three types of cone cell in the retina, sensitive to long, medium and short wavelengths — loosely, red, green and blue. Colour blindness is almost always a fault in the long or medium cones, which is why the overwhelming majority of cases are red-green rather than blue-yellow. The condition comes in degrees: an anomaly, where a cone type is present but shifted in sensitivity — protanomaly for the red cone, deuteranomaly for the green — and a full absence, protanopia or deuteranopia. Complete absence of colour vision, monochromacy, is rare, and a monochromat may see nothing at all on an Ishihara plate. The genetics matter for exams because the genes for the red and green pigments sit on the X chromosome, so red-green colour blindness is an X-linked recessive condition: a man with one affected X is affected, while a woman needs both, which makes the condition several times commoner in men and makes carrier mothers the usual route of transmission. That is why an affected mother and an unaffected father produce affected sons and carrier daughters — a pattern UPSC has asked directly.

State PCS 2025 Q80 — Science & Technology PYQ with Answer

State PCS 2025 Q80 — Science & Technology PYQ with Answer