**Question:** WHO definition of blindness, Visual acuity -

A. 20/200 or worse
B. Perception of light
C. Best corrected visual acuity (BCVA) of 6/60 or less
D. BCVA of 6/60 to 6/18

**Correct Answer:** C. Best corrected visual acuity (BCVA) of 6/60 or less

**Core Concept:** Visual acuity is a measure of an individual's ability to see distant objects clearly. The WHO defines blindness as the presence of low vision, in which the individual's visual acuity is significantly impaired, but not to the extent that they are classified as legally blind.

**Why the Correct Answer is Right:** The correct answer (C) refers to the World Health Organization (WHO) definition of blindness, which is characterized by a best corrected visual acuity (BCVA) of 6/60 or less. This means that an individual with visual acuity at or below 6/60 can't see objects clearly at 6 meters, which is the standard distance for visual acuity measurement. BCVA refers to the best possible visual acuity achieved when the person is wearing corrective lenses (e.g., glasses or contact lenses) to correct refractive errors.

**Why Each Wrong Option is Incorrect:**

A. 20/200 or worse (6/60 or better) is the threshold for low vision, not blindness. This option is incorrect because it doesn't meet the WHO definition of blindness.

B. Perception of light refers to the ability to detect light or darkness, not a numerical value for visual acuity. It is not a definition of blindness but rather a condition like no light perception (NLP).

D. BCVA of 6/60 to 6/18 implies that the individual can still see objects more clearly than those with complete blindness (WHO definition). This option is incorrect as it doesn't meet the WHO criteria for blindness.

**Clinical Pearl:**

Understanding the WHO definition of blindness is essential for healthcare providers, as it helps in determining patient eligibility for appropriate interventions, rehabilitation, and resource allocation to improve visual function and quality of life for those with severe visual impairment.