OP1.1-5 | Visual Foundations and Refraction — Practice Quiz

Correct. The cornea contributes ~43 D of the total ~60 D, making it the dominant refracting surface. Its power is fixed; the lens provides the variable component via accommodation.

The cornea contributes approximately 43 D (fixed) and the crystalline lens approximately 17–20 D (variable via accommodation). The cornea is the single largest contributor.

The cornea contributes ~43 D — the largest fixed component. The crystalline lens contributes ~17–20 D but is the variable component used in accommodation.

Correct. The optic chiasma is where nasal (temporal-field) fibres from both eyes cross. Compression here — classically by a pituitary adenoma — produces bitemporal hemianopia.

At the optic chiasma, nasal fibres (which carry the temporal field from each eye) decussate. A lesion here interrupts nasal fibres from both eyes, producing bitemporal hemianopia — the classic field defect of a pituitary tumour.

Bitemporal hemianopia localises to the optic chiasma, where nasal fibres from both eyes cross. A left optic nerve lesion causes monocular loss; optic tract/radiation lesions cause homonymous defects.

Correct. Myopia is corrected with a concave (minus, diverging) lens, which diverges the incoming rays and shifts the focal point back to the retina.

Myopia results from the focal point falling anterior to the retina (eye too long or too powerful). A concave (diverging, minus) lens moves the focal point posteriorly to land on the retina.

This is myopia — the focal point is anterior to the retina. A convex (plus) lens would shift it further forward, worsening the blur. A concave (minus) lens is needed.

Correct. Presbyopia begins around age 40–45 as the crystalline lens loses elasticity. Near vision is impaired (reading difficulty) but distance vision is preserved — the hallmark of presbyopia.

Presbyopia is the age-related loss of accommodative amplitude, typically symptomatic from age 40–45. The lens hardens progressively, reducing the ability to increase its curvature for near focus. Distance vision is unaffected.

Presbyopia is the correct answer. New-onset near-vision difficulty at age 40–45 in someone with previously normal vision is presbyopia, not hypermetropia (which causes near and sometimes distance blur from childhood).

Correct. Snellen notation: 6/60 means the patient reads at 6 m the letter that a normal eye resolves at 60 m. This is near the legal threshold for significant visual impairment.

Snellen VA notation: numerator = test distance (6 m); denominator = distance at which a normal eye can resolve that letter (subtending 5 arcmin). 6/60 means the patient reads at 6 m what a normal eye resolves at 60 m — indicating significant visual impairment.

In Snellen notation, 6/60 means the patient's eye reads at 6 m what a normal eye resolves at 60 m — not a percentage. Normal vision is 6/6.

Correct. Pinhole improvement indicates that the visual loss is refractive in origin. Organic causes (macular, optic nerve, amblyopia) do NOT improve with a pinhole.

A pinhole restricts light entry to the central paraxial rays, eliminating the blur caused by refractive error. Improvement with pinhole means the blur is optical (refractive), not organic (retinal/neural). Non-improvement suggests a structural or neural cause.

Pinhole improvement means the blur is refractive — correctable with lenses. Organic causes (macula, optic nerve) and amblyopia do not improve with a pinhole. This is the most important clinical distinction in VA testing.

Correct. Keratoconus is an absolute contraindication to laser refractive surgery. Pre-operative corneal topography is mandatory to detect even subclinical keratoconus before LASIK.

Keratoconus is a progressive corneal thinning disorder. Laser ablation on an already weakened, ectatic cornea risks precipitating catastrophic post-LASIK ectasia with severe, irreversible visual loss. Corneal topography must be performed before every laser refractive surgery evaluation to screen for keratoconus — including subclinical (forme fruste) cases.

Keratoconus is the key contraindication to LASIK. Ablating a progressively thinning cornea can cause devastating post-operative ectasia. The other options are within acceptable parameters for refractive surgery.

Correct. Excimer laser ablation removes central corneal stroma to flatten the cornea, reducing its plus power and correcting myopia. This is purely photochemical (photoablation) — not thermal.

The 193 nm excimer laser breaks molecular bonds (photoablation) with negligible thermal spread. For myopia, central stromal tissue is removed, flattening the anterior corneal surface and reducing its refracting power — shifting the focal point posteriorly onto the retina.

Excimer laser causes photoablation of corneal stroma — breaking molecular bonds without significant heat. For myopia, central tissue removal flattens the cornea and reduces its refracting power.

Correct. Amblyopia is diagnosed when VA is reduced, there is no improvement with the best optical correction, and structural examination is normal. The critical period for treatment is before age 7.

Amblyopia is reduced BCVA not explained by structural ocular pathology, caused by abnormal visual experience during the critical period (birth to ~7 years). Normal fundus + no improvement with optimal refraction + no structural cause = amblyopia. The diagnosis is one of exclusion.

The key features — reduced VA not improved by pinhole or spectacles, plus a structurally normal eye — define amblyopia. Optic neuritis, corneal opacity, and retinal detachment all produce structural findings.

Correct. Anisometropic amblyopia occurs when a significant inter-ocular refractive difference causes the more ametropic eye to send chronically blurred signals, leading to cortical suppression.

Anisometropia means unequal refractive errors between the two eyes. The brain receives a consistently blurred image from the more ametropic eye and suppresses it, leading to amblyopia in that eye. This type is insidious — there is no obvious squint to alert parents or clinicians.

Anisometropic amblyopia is caused by unequal refractive errors between the eyes. Strabismic = caused by squint; deprivation = caused by obstruction (cataract, ptosis); ametropic = high bilateral refractive error.

Correct. Occlusion therapy works by forcing visual experience through the amblyopic eye during the critical period, when the visual cortex retains neuroplasticity and can strengthen under-used synaptic connections.

Patching exploits cortical plasticity during the critical period (maximally effective before age 7). By occluding the dominant eye, all form vision is forced through the amblyopic eye, driving synaptic strengthening in the under-used ocular dominance columns of V1. The window narrows after age 7 and closes by puberty.

Patching works through neuroplasticity, not muscle training or deliberate weakening. By forcing all vision through the amblyopic eye during the critical period, cortical synapses are strengthened and VA improves.

Correct. Cylindrical (toric) lenses correct astigmatism by providing differential refraction along a specific axis, compensating for the unequal corneal curvature.

Astigmatism arises when the refracting power differs in different meridians (most commonly due to a non-spherical corneal surface). A cylindrical lens has power in only one meridian, which can be aligned to neutralise the excess or deficit power in the steeper or flatter corneal meridian.

Cylindrical lenses are used to correct astigmatism. Spherical lenses correct myopia or hypermetropia uniformly in all meridians; bifocal lenses address presbyopia.