BPSC 70th CCE Re-exam Prelims General Studies · Question 80 of 142

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.

• (b)Haemoglobin level — 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)IQ — 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 density — 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.

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.

Questions of the form 'test X is related with Y' are answered by attaching each named measurement to its own instrument, and the safeguard is to make sure no two options can claim the same one. Here the four measurements come from four different disciplines — ophthalmology, haematology, psychology and radiology — so a candidate who can name one instrument per discipline has the question. Build the pairs deliberately, because BPSC returns to them: Ishihara plates for colour vision, Snellen chart for visual acuity, tonometry for intraocular pressure and glaucoma, sphygmomanometer for blood pressure, ECG for heart rhythm, EEG for brain activity, spirometer for lung function, DEXA for bone density, Mantoux for tuberculosis exposure, ELISA for antibodies. A second discriminator is available here for free: the Ishihara test needs no equipment beyond a book of printed plates and good lighting, and none of the other three measurements can be made that way.

• The Ishihara test detects red-green colour deficiencies and was first published in 1917 by Shinobu Ishihara, a professor at the University of Tokyo.
• It uses pseudo-isochromatic plates — circles of coloured dots of varying size and shade concealing a figure; on the best-known plate, normal vision reads 74 while red-green deficient vision reads 21.
• The full test has 38 plates, with shorter 10, 14 and 24-plate versions; plate types include demonstration, transformation, vanishing, hidden-digit, diagnostic and tracing plates.
• Red-green colour blindness is an X-linked recessive condition, which is why it is far commoner in men than in women; protanopia and deuteranopia name the loss of the red and green cone systems respectively.
• The other measurements and their instruments: haemoglobin by haemoglobinometer or automated counter, IQ by Stanford-Binet, Wechsler or Raven's matrices, and bone mineral density by DEXA scan.

• Assuming a colour-based test must be about blood because the old Sahli haemoglobinometer also matched colours. The purpose, not the technique, defines the test.
• Confusing the Ishihara plates with Raven's Progressive Matrices. Both are printed pattern tests, but Raven's is a reasoning test built to be independent of colour.
• Treating colour blindness as an all-or-nothing condition. Anomalies such as protanomaly and deuteranomaly are far commoner than complete dichromacy, and monochromacy is rare.

BPSC asks named tests, instruments and diseases as one-line identifications, drawing its wrong options from unrelated branches of medicine so that a single correct pairing settles the question — the 70th CCE paper of December 2024 did the same with a disease and its common name. UPSC does not ask what a test is called; it asks the biology underneath, most often the inheritance pattern of the condition being tested for.

• practice — not a real PYQ

  Bone mineral density is most commonly measured by

  • (a)An Ishihara plate test
  • (b)A DEXA scan
  • (c)A spirometer
  • (d)A tonometer

  Answer(b) A DEXA scan — dual-energy X-ray absorptiometry, used to diagnose osteoporosis. A spirometer measures lung function and a tonometer measures intraocular pressure.

• practice — not a real PYQ

  Red-green colour blindness is inherited as

  • (a)An autosomal dominant trait
  • (b)An autosomal recessive trait
  • (c)An X-linked recessive trait
  • (d)A Y-linked trait

  Answer(c) An X-linked recessive trait — the red and green pigment genes lie on the X chromosome, which is why the condition is far commoner in men and is usually transmitted through carrier mothers.