Colour blindness is due to a problem with?

Daman & Diu Staff Nurse - 2018

Applied Physiology

Easy

Appeared in: Daman & Diu Staff Nurse - 2018

Explanation

Cones are the photoreceptor cells located in the retina that are responsible for color vision (photopic vision).
There are three types of cones, each sensitive to different light wavelengths: red, green, and blue.
Color blindness occurs when one or more of these cone types are absent, non-functional, or detect an abnormal spectrum of light, leading to an inability to distinguish between certain colors.
The most common form is red-green color blindness, which results from a defect in the red or green cones.

Option A: The lens is responsible for focusing light onto the retina. It is not involved in the perception of color.
Option B: Aqueous humour is the fluid that fills the anterior and posterior chambers of the eye. Its function is to maintain intraocular pressure and provide nutrients to the lens and cornea, not to perceive light or color.
Option D: Rods are photoreceptor cells that are highly sensitive to light and are responsible for vision in dim light (scotopic vision). They detect shades of black, white, and gray, but not color.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Physiology of vision and pathophysiology of color blindness as background academic context rather than a clinical decision trigger.
Nurses should be aware that the Ishihara test, which uses plates with colored dots, is a common and rapid method for screening for red-green color blindness.
When providing patient education or instructions, especially regarding medications (e.g., 'take the red pill'), nurses must avoid relying solely on color-coding for patients with known color blindness. Using clear text, symbols, or different shapes is a crucial patient safety measure.
What if? If a patient complains of difficulty seeing in the dark or at night, the problem is likely with the rods (nyctalopia), not the cones. This would prompt a different line of investigation, possibly including an assessment of Vitamin A status.

How to Approach the Question

First, identify the key concept in the question, which is the physiological basis of 'color blindness'.
Recall the basic anatomy of the eye and the function of its different parts, particularly the retina.
Differentiate between the two main types of photoreceptor cells in the retina: rods and cones.
Remember that cones are responsible for color vision (photopic vision) and function in bright light, while rods are for low-light vision (scotopic vision) and see in black and white.
Logically connect the inability to see 'color' (color blindness) with a defect in the cells responsible for color perception, which are the cones.
Eliminate the other options by recalling their functions: the lens focuses light, aqueous humor maintains pressure, and rods are for night vision.

Concept Tested & Keywords

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E6 Physiology Guyton 4SAE Part 3 pp. 156-158, 150-151, 155-157