20 Ocular Anatomy & Physiology Practice Questions & Answers

1. 1. Which corneal layer is acellular and, once damaged, heals by scar formation rather than regenerating?

   • A.Descemet's membrane, secreted by the endothelium
   • B.The stroma, the thickest layer of the cornea
   • C.The corneal epithelium, the outermost layer
   • D.Bowman's layer, lying beneath the epithelium✓ Answer

   Bowman's layer is a condensed acellular sheet of collagen with no cells to repopulate it, so any breach through it leaves a permanent scar. The epithelium regenerates fully in about a week, Descemet's membrane is continuously resecreted by the endothelium, and the stroma repairs itself with keratocyte activity even though that repair may leave haze.

   Source: ABO-NCLE CLRE Basic Test Specifications, Anterior Segment — corneal structure; standard contact lens texts on corneal anatomyReport a problem with this question

2. 2. In the open eye, what is the cornea's principal source of the oxygen it needs?

   • A.Vessels of the palpebral conjunctiva lining the eyelids
   • B.Atmospheric oxygen dissolved in the precorneal tear film✓ Answer
   • C.Blood delivered by capillaries within the corneal stroma
   • D.Oxygen carried in the aqueous humor across the endothelium

   The cornea is avascular, so with the lids open it takes essentially all of its oxygen from the atmosphere after that oxygen dissolves into the tear film covering it. The palpebral vessels become the supply only when the lids close, which is why the closed-eye oxygen level is much lower and overnight wear is the higher-risk condition; the aqueous supplies mainly glucose rather than oxygen.

   Source: ABO-NCLE CLRE Basic Test Specifications, corneal metabolism and oxygen supplyReport a problem with this question

3. 3. A patient in low-oxygen lenses develops stromal edema. What metabolic change explains the swelling?

   • A.Increased aerobic metabolism raises stromal collagen production sharply
   • B.Reduced tear osmolarity floods the epithelium with excess surface fluid
   • C.Endothelial cell division thickens the posterior corneal surface layer
   • D.Anaerobic glycolysis accumulates lactate, drawing water into the stroma✓ Answer

   When oxygen is short the epithelium shifts to anaerobic glycolysis, and the lactate it produces diffuses back into the stroma and raises the osmotic pull there, so water moves in and the tissue swells. The endothelial pump cannot clear that extra load, which is why persistent hypoxia progresses to striae and haze rather than settling on its own.

   Source: ABO-NCLE CLRE Basic Test Specifications, corneal metabolism, edema; contact lens texts on hypoxic corneal responseReport a problem with this question

4. 4. Which tear film layer is produced by the goblet cells of the conjunctiva, and what does it accomplish?

   • A.The lipid layer, which slows evaporation from the tear surface
   • B.The lacrimal lake, which stores tears along the lower lid margin
   • C.The mucin layer, which lets tears wet the corneal surface✓ Answer
   • D.The aqueous layer, which carries oxygen and antibacterial enzymes

   Conjunctival goblet cells secrete mucin, which coats the naturally water-repelling epithelium and lowers surface tension so the watery portion of the tears can spread evenly instead of beading up. The lipid layer comes from the meibomian glands and the aqueous layer from the lacrimal glands, and the lacrimal lake is a reservoir rather than a layer.

   Source: ABO-NCLE CLRE Basic Test Specifications, Adnexa — tear film layers and glandsReport a problem with this question

5. 5. A lens wearer has chronic meibomian gland dysfunction. Which change in the tear film follows most directly?

   • A.A deficient mucin layer, so tears bead up on the corneal surface
   • B.A deficient lipid layer, so tears evaporate faster between blinks✓ Answer
   • C.Increased lysozyme output, so the tear film resists bacteria better
   • D.Reduced aqueous volume, so the tear reservoir at the lid empties

   The meibomian glands sit within the tarsal plate and supply the outer lipid layer, so when they are obstructed or inflamed the oily covering thins and the tears below it evaporate much more quickly. That evaporative instability is why lens wearers with meibomian disease report end-of-day dryness even when total tear volume is adequate.

   Source: ABO-NCLE CLRE Basic Test Specifications, Adnexa — eyelid glands; Pathology — blepharitis and meibomian gland dysfunctionReport a problem with this question

6. 6. What does tear break-up time assess, and what does a short break-up time imply for a prospective lens wearer?

   • A.Tear film stability; an unstable film predicts dryness and poor wetting✓ Answer
   • B.Corneal sensitivity; a dull response predicts delayed epithelial healing
   • C.Tear drainage rate; rapid outflow predicts pooling at the medial canthus
   • D.Total tear production; low volume predicts reflex tearing on insertion

   Break-up time measures how long an intact tear film holds together between blinks, so it reports stability rather than quantity, and early break-up usually points to a failing lipid or mucin component. A patient whose film breaks up quickly is more likely to experience surface dryness, deposits and poor lens wetting, so the tear film should be addressed before fitting.

   Source: ABO-NCLE CLRE Basic Test Specifications, Adnexa — tear film break-up timeReport a problem with this question

7. 7. Why does the tarsal conjunctiva of the upper lid show papillae rather than follicles in lens-related irritation?

   • A.It is covered by the lipid layer, so any allergic reaction is neutralized there
   • B.It is tightly bound to the tarsal plate, so mechanical insult raises papillae✓ Answer
   • C.It lies farthest from the limbal vessels, so its response is always mechanical
   • D.It contains no lymphoid tissue at all, so a follicular response cannot form

   Because the palpebral conjunctiva is firmly anchored to the underlying tarsal plate, inflammatory swelling is tethered around the vessels and rises as discrete papillae with a central vascular core. Follicles are collections of lymphoid tissue typical of viral or chlamydial disease and are seen more often in the looser inferior fornix, so lens-induced change on the upper tarsus reads as papillary.

   Source: ABO-NCLE CLRE Basic Test Specifications, Adnexa — conjunctiva; Pathology — giant papillary conjunctivitisReport a problem with this question

8. 8. A soft lens wearer reports itching that is worst on lens removal, with stringy mucus and a lens that rides high and moves excessively. Everting the upper lid shows enlarged papillae. What is this presentation?

   • A.Adenoviral conjunctivitis spreading from the fellow eye and lid margin
   • B.Superior limbic keratoconjunctivitis from a preservative sensitivity
   • C.Giant papillary conjunctivitis from friction and deposit-borne antigen✓ Answer
   • D.Acanthamoeba keratitis acquired from water exposure during showering

   Itching that peaks on removal, ropy mucus, superior decentration with excess movement and enlarged upper tarsal papillae together describe giant papillary conjunctivitis, driven by repeated lid friction against the lens plus antigen held in surface deposits. It is managed by reducing that antigen load and friction, through more frequent replacement, preservative-free care and a period out of lenses.

   Source: ABO-NCLE CLRE Basic Test Specifications, Pathology — giant papillary conjunctivitis; objective findingsReport a problem with this question

9. 9. Which feature most strongly suggests an infiltrate is infectious rather than sterile?

   • A.A hazy ring at the limbus that clears within hours of lens removal
   • B.A small peripheral lesion with intact epithelium and mild irritation
   • C.A central lesion with an overlying epithelial defect and marked pain✓ Answer
   • D.Several faint arcuate opacities near the limbus with no discomfort

   Infectious keratitis tends to be larger and more central, with the epithelium broken over the lesion so it stains, and with pain, discharge and an anterior chamber reaction out of keeping with a sterile response. Sterile infiltrates are typically small, peripheral, multiple and comparatively quiet, but because the distinction is not reliable at the chair, every infiltrate should be treated as infectious until the prescriber says otherwise.

   Source: ABO-NCLE CLRE Basic Test Specifications, Pathology — corneal infiltrates and ulcers, microbial keratitisReport a problem with this question

10. 10. A lens wearer telephones with a red, painful, light-sensitive eye and discharge that began overnight. What is the appropriate response?

    • A.Advise switching to a fresh lens and a new solution, then monitoring wear
    • B.Advise removing the lens and arrange same-day evaluation by the prescriber✓ Answer
    • C.Advise removing the lens, patching the eye, and rechecking in the morning
    • D.Advise continuing wear on a shortened schedule and calling back in a week

    Redness with pain, photophobia and discharge in a lens wearer is the classic warning picture for microbial keratitis, which can threaten sight within a day, so the lens comes out and the patient is seen by the prescriber urgently. Patching traps organisms against a warm moist cornea and worsens the risk, and a technician neither diagnoses nor treats the condition.

    Source: ABO-NCLE CLRE Basic Test Specifications, Pathology — microbial keratitis; objective findings and referralReport a problem with this question

11. 11. Why does chronic hypoxia in lens wear lead to polymegethism of the corneal endothelium?

    • A.Endothelial cells divide rapidly, so crowding forces them into varied sizes
    • B.Endothelial cells absorb lactate directly and swell to differing diameters
    • C.Endothelial cells migrate forward into the stroma and lose their alignment
    • D.Endothelial cells do not divide, so survivors spread unevenly to cover gaps✓ Answer

    The human corneal endothelium is a single layer that does not replace lost cells by division, so when chronic oxygen deprivation stresses the tissue the remaining cells enlarge and spread to keep the barrier continuous. That gives the variation in cell size called polymegethism, alongside the variation in shape called pleomorphism, and both are markers of long-term metabolic stress rather than acute injury.

    Source: ABO-NCLE CLRE Basic Test Specifications, Anterior Segment — endothelium; contact lens texts on chronic hypoxic changeReport a problem with this question

12. 12. What role does the limbus play that makes damage there particularly serious for a lens wearer?

    • A.It holds the stem cells that renew the corneal epithelium continuously✓ Answer
    • B.It holds the nerve plexus that triggers the blink reflex on lens contact
    • C.It holds the goblet cells that supply the mucin layer of the tear film
    • D.It holds the accessory glands that produce the basal aqueous tear flow

    The limbus is the narrow junction between cornea and sclera and houses the stem cell population that repopulates the corneal epithelium throughout life. If lens bearing, chronic hypoxia or mechanical trauma depletes those cells, conjunctival tissue and vessels can grow across the cornea in their place, which is why limbal hyperemia and early vessel ingrowth are treated as signals to increase oxygen and reduce wear.

    Source: ABO-NCLE CLRE Basic Test Specifications, Anterior Segment — limbus, vascularizationReport a problem with this question

13. 13. A rigid lens wearer shows arcuate staining at the 3 and 9 o'clock positions of the cornea. What mechanism produces it?

    • A.Endothelial pump failure allowing fluid to collect at the horizontal edge
    • B.An immune reaction to a solution preservative held under the lens
    • C.Trapped air beneath a steep lens pooling dye in surface indentations
    • D.Peripheral drying where the lid fails to resurface tears beside the lens✓ Answer

    A rigid lens sits proud of the surrounding cornea, so an incomplete or infrequent blink cannot carry tears smoothly across the exposed strip on either side and that band dries and stains. Poor blinking, a lens that rides low or too flat, and a thick or poorly blended edge all worsen it, so the answer lies in blink retraining and lens edge or position changes rather than treating an infection.

    Source: ABO-NCLE CLRE Basic Test Specifications, Adnexa — blink rate and effect; 3 and 9 o'clock stainingReport a problem with this question

14. 14. Which pair of features distinguishes contact-lens-associated red eye (CLARE) from a contact lens peripheral ulcer (CLPU)?

    • A.CLARE spares the epithelium and follows overnight wear; CLPU breaks it and scars✓ Answer
    • B.CLARE stems from a solution sensitivity; CLPU stems from a tight lens seal
    • C.CLARE breaks the epithelium and follows overnight wear; CLPU spares it and scars
    • D.CLARE is a single arcuate lesion under the upper lid; CLPU is a diffuse reaction

    CLARE is an acute, usually unilateral inflammatory reaction that wakes a patient during or after overnight wear, with diffuse peripheral infiltrates and an intact epithelium, and it settles once the lens is out. A peripheral ulcer is a small round focal lesion whose epithelium is broken and therefore stains, and it heals leaving a small scar, so the state of the epithelium is the practical discriminator.

    Source: ABO-NCLE CLRE Basic Test Specifications, Pathology — contact lens related conditions; CLARE, CLPUReport a problem with this question

15. 15. Why is the cornea unusually painful when its epithelium is abraded?

    • A.Pain fibers of the optic nerve run forward from the retina to the epithelium
    • B.Free nerve endings from the trigeminal nerve are dense in the epithelium✓ Answer
    • C.Sensory branches of the facial nerve terminate throughout the epithelium
    • D.Autonomic fibers from the ciliary muscle spread outward into the epithelium

    The cornea is supplied by the ophthalmic division of the trigeminal nerve, and its free nerve endings are packed most densely just beneath and within the epithelium, making it the most richly innervated tissue in the body. Stripping that epithelium exposes those endings directly, which produces intense pain and reflex tearing and blepharospasm; the facial nerve drives lid closure but carries no corneal sensation.

    Source: ABO-NCLE CLRE Basic Test Specifications, Anterior Segment — corneal innervation and sensitivityReport a problem with this question

16. 16. How does corneal transparency depend on the water content of the stroma?

    • A.Higher water content raises the refractive index and improves light transmission
    • B.Full hydration keeps collagen fibrils separated so light passes without scatter
    • C.Water content has no bearing because the stroma has no cells to scatter light
    • D.Relative dehydration keeps collagen fibrils evenly spaced so light passes cleanly✓ Answer

    Stromal collagen fibrils are thin and arranged with a regular spacing smaller than the wavelength of light, so their scattered wavelets cancel and the tissue looks clear. The endothelial pump holds the stroma in a relatively deturgesced state to preserve that spacing, and when hypoxia or pump failure lets water in the lattice is disrupted, scatter rises and the cornea hazes.

    Source: ABO-NCLE CLRE Basic Test Specifications, Anterior Segment — transparency, dehydration, edemaReport a problem with this question

17. 17. A patient with well-controlled diabetes asks about contact lenses. Which ocular consequence of the disease most affects the fitter's plan?

    • A.Reduced pupil diameter in dim light with resulting loss of night vision
    • B.Reduced lid tension with a lens that drops below the lower lid margin
    • C.Reduced scleral rigidity with a landing zone that flattens over the day
    • D.Reduced corneal sensation with slower epithelial healing after any insult✓ Answer

    Diabetes blunts corneal sensitivity and weakens epithelial adhesion and repair, so a wearer may not feel a developing problem and any abrasion is slower to close and more prone to infection. That combination calls for conservative wear schedules, no overnight wear and closer follow-up rather than an outright refusal, and refraction can also fluctuate with blood glucose.

    Source: ABO-NCLE CLRE Basic Test Specifications, Pathology — diseases affecting contact lens wear (diabetes)Report a problem with this question

18. 18. How does a pterygium differ from a pinguecula on examination?

    • A.A pterygium forms within the corneal stroma without touching the conjunctiva
    • B.A pterygium sits on the tarsal plate and lifts the lid away from the eye
    • C.A pterygium extends across the limbus onto the corneal surface✓ Answer
    • D.A pterygium arises at the lid margin and grows outward toward the lashes

    Both are degenerative conjunctival changes that appear in the exposed interpalpebral zone, but only the pterygium invades the cornea by crossing the limbus, typically as a wing-shaped fold from the nasal side. A pinguecula stays on the bulbar conjunctiva; either can raise the surface enough to disturb tear spread and soft lens edge comfort, and progressive corneal encroachment warrants referral.

    Source: ABO-NCLE CLRE Basic Test Specifications, Pathology — pterygium, pingueculaReport a problem with this question

19. 19. Which description matches keratoconus as the fitter would recognize it?

    • A.Flattening of the central cornea that leaves the periphery relatively steep
    • B.Deposition of lipid in a ring at the periphery that spares the visual axis
    • C.Endothelial failure that floods the epithelium and raises fluid-filled bullae
    • D.Non-inflammatory thinning that steepens and distorts the corneal surface✓ Answer

    Keratoconus is a progressive non-inflammatory ectasia in which the stroma thins and the cornea bulges, usually below and temporal to center, giving steep irregular readings, a scissoring retinoscopy reflex and vision that spectacles cannot fully correct. Bullous keratopathy is the endothelial failure picture, a lipid ring at the periphery describes arcus, and central flattening with a steeper periphery describes the oblate cornea after refractive surgery.

    Source: ABO-NCLE CLRE Basic Test Specifications, Pathology — keratoconusReport a problem with this question

20. 20. A patient panics that a soft lens has slipped behind the eye. What anatomical fact should reassure them?

    • A.The conjunctiva folds back at the fornix, sealing the space around the globe✓ Answer
    • B.The orbital septum closes behind the globe, blocking any object from passing
    • C.The lens dissolves within minutes once it loses contact with the tear film
    • D.The extraocular muscles form a ring that a displaced lens cannot slip through

    The bulbar conjunctiva covering the globe is continuous with the palpebral conjunctiva lining the lids, and the reflection between them at the superior and inferior fornix forms a closed pocket, so nothing can travel behind the eye. A displaced lens has simply lodged in a fornix, most often the deeper superior one, and it is found by everting or lifting the lid.

    Source: ABO-NCLE CLRE Basic Test Specifications, Adnexa — conjunctiva, fornixReport a problem with this question

Practice questions based on the ABO-NCLE Contact Lens Registry Exam content domains and standard contact lens references, together with the FTC Contact Lens Rule and FDA device classification. This site is not affiliated with or endorsed by ABO-NCLE. Contact lens fitting is regulated state by state — scope of practice, supervision and licensure requirements vary, so confirm your own state's rules and work under the prescriber's direction. Verify current exam requirements with ABO-NCLE before testing. About the ABO-NCLE exams →