Diseases of the Cornea — MCQs

Diseases of the Cornea Indian Medical PG Practice Questions and MCQs

Question 291: Corneal thickness is measured using which diagnostic tool?

A. Pachymetry (Correct Answer)
B. Keratometer
C. CT scan
D. Aesthesiometry

Explanation: **Explanation:** **Corneal Pachymetry** is the gold standard diagnostic tool used to measure the thickness of the cornea. This measurement is clinically vital because corneal thickness influences the accuracy of Intraocular Pressure (IOP) readings (thinner corneas lead to underestimated IOP) and is a critical screening parameter for refractive surgeries like LASIK and the management of Keratoconus. **Analysis of Options:** * **Pachymetry (Correct):** It can be performed using **ultrasound** (contact method) or **optical** methods (non-contact, e.g., Pentacam or OCT). The average central corneal thickness (CCT) is approximately **540–560 μm**. * **Keratometer:** This instrument measures the **curvature** of the anterior corneal surface. It is used to calculate the power of Intraocular Lenses (IOL) and to diagnose astigmatism. * **CT Scan:** While useful for orbital trauma or tumors, it lacks the resolution required to measure microscopic corneal dimensions. * **Aesthesiometry:** This measures **corneal sensitivity** (nerve function). The Cochet-Bonnet aesthesiometer is the classic tool used for this purpose, often to evaluate viral keratitis (e.g., Herpes Simplex) or neurotrophic ulcers. **High-Yield Clinical Pearls for NEET-PG:** * **Goldmann Applanation Tonometry (GAT):** It is calibrated for a CCT of 520 μm. If the cornea is thicker, the IOP reading will be falsely high. * **Keratoconus:** Characterized by progressive **corneal thinning** and steepening. * **Fuch’s Dystrophy:** Leads to corneal edema, resulting in an **increased** pachymetry reading. * **Refractive Surgery:** A minimum residual stromal bed of **250–300 μm** must remain after LASIK to prevent corneal ectasia.

Question 292: Prominent corneal nerves are seen in all conditions except?

A. Ectodermal dysplasia
B. Ichthyosis
C. Refsum's syndrome
D. Macular dystrophy (Correct Answer)

Explanation: **Explanation:** The presence of **prominent corneal nerves** is a classic clinical sign in ophthalmology, often associated with specific systemic and ocular conditions. **Why Macular Dystrophy is the correct answer:** Macular corneal dystrophy is a stromal dystrophy characterized by the accumulation of acid mucopolysaccharides. It typically presents with diffuse stromal haze and focal gray-white opacities. It **does not** cause enlargement or increased visibility of corneal nerves. In fact, most corneal dystrophies (except occasionally Lattice dystrophy) are not associated with prominent nerves. **Analysis of Incorrect Options (Conditions with Prominent Nerves):** * **Ectodermal Dysplasia:** A group of genetic disorders affecting skin, hair, and nails; prominent corneal nerves are a recognized ocular feature. * **Ichthyosis:** Specifically X-linked ichthyosis is frequently associated with thickened corneal nerves and deep stromal opacities. * **Refsum’s Syndrome:** A metabolic disorder (phytanic acid storage disease) that presents with retinitis pigmentosa and prominent corneal nerves. **High-Yield Clinical Pearls for NEET-PG:** To remember the causes of prominent corneal nerves, use the mnemonic **"MEN'S HIRE"**: * **M:** **M**ultiple Endocrine Neoplasia (MEN) Type 2B (Most common association) * **E:** **E**ctodermal dysplasia / **E**dema (Corneal) * **N:** **N**eurofibromatosis Type 1 (NF-1) * **S:** **S**iogren’s syndrome * **H:** **H**ansen’s disease (Leprosy) * **I:** **I**chthyosis * **R:** **R**efsum’s syndrome * **E:** **E**hlers-Danlos syndrome / **E**xophthalmos (Grave’s disease) **Key Fact:** The most common systemic association tested in exams for thickened corneal nerves is **MEN Type 2B** (associated with Medullary Thyroid Carcinoma and Pheochromocytoma).

Question 293: Which organism does not invade an intact cornea?

A. Gonococci
B. C. diphtheriae
C. Meningococci
D. Pseudomonas (Correct Answer)

Explanation: **Explanation:** The corneal epithelium acts as a formidable mechanical barrier against most microorganisms. For a pathogen to cause a corneal ulcer, there usually must be a prior breach in this epithelium (e.g., trauma, contact lens wear, or dry eye). **Why Pseudomonas is the correct answer:** *Pseudomonas aeruginosa* is a highly virulent gram-negative rod, but it **cannot** penetrate a healthy, intact corneal epithelium. It requires a pre-existing epithelial defect to initiate infection. Once inside the stroma, however, it produces proteases and collagenases that cause rapid liquefactive necrosis (the "soupy" cornea). **Why the other options are incorrect:** There are five specific organisms known for their unique ability to penetrate a completely **intact** corneal epithelium. These are often remembered by the mnemonic **"CHNL-G"**: 1. **C**orynebacterium diphtheriae (Option B) 2. **H**aemophilus aegyptius (Koch-Weeks bacillus) 3. **N**eisseria gonorrhoeae (Option A) 4. **L**isteria monocytogenes 5. **G**onococci/Meningococci (Option C) These organisms possess specific surface proteins or enzymes that allow them to bypass the epithelial defense without prior trauma. **High-Yield Clinical Pearls for NEET-PG:** * **Pseudomonas:** The most common cause of bacterial corneal ulcers in **contact lens users**. It is characterized by a greenish-yellow discharge. * **Neisseria gonorrhoeae:** Can cause hyperacute conjunctivitis leading to rapid corneal perforation due to its ability to invade intact tissue. * **Acanthamoeba:** Another organism that typically requires a breach (often associated with contaminated contact lens solution) but is notorious for causing a "ring-shaped infiltrate."

Question 294: The thickness of the cornea is measured by using which instrument?

A. Schiotz tonometer
B. Keratometer
C. Pachymeter (Correct Answer)
D. Aesthesiometer

Explanation: **Explanation:** The measurement of corneal thickness is known as **Pachymetry**, and the instrument used is a **Pachymeter**. This is a critical clinical measurement because corneal thickness influences the accuracy of Intraocular Pressure (IOP) readings and determines candidacy for refractive surgeries like LASIK. **Why the correct answer is right:** * **Pachymeter:** It measures the distance between the corneal epithelium and endothelium. There are two main types: **Optical pachymeters** (using a slit lamp) and **Ultrasound pachymeters** (more common, using high-frequency sound waves). The average central corneal thickness (CCT) is approximately **540–560 μm**. **Analysis of incorrect options:** * **Schiotz tonometer:** Used to measure **Intraocular Pressure (IOP)** based on the principle of indentation. It is no longer the gold standard (which is Goldmann Applanation Tonometry). * **Keratometer:** Used to measure the **curvature** of the anterior corneal surface. It is essential for calculating IOL power and fitting contact lenses. * **Aesthesiometer:** Used to measure **corneal sensitivity** (e.g., Cochet-Bonnet aesthesiometer). Reduced sensitivity is seen in conditions like Herpes Simplex Keratitis or Diabetes. **High-Yield Clinical Pearls for NEET-PG:** 1. **IOP Correction:** If the cornea is thinner than average, the measured IOP is falsely low; if thicker, the IOP is falsely high. 2. **Keratoconus:** A thin cornea (usually <450–500 μm) with apical scarring is a hallmark of progressive keratoconus. 3. **Specular Microscopy:** While primarily used to count endothelial cells, it can also provide a measurement of corneal thickness.

Question 295: What is the earliest symptom to occur in a corneal ulcer?

A. Pain (Correct Answer)
B. Photophobia
C. Loss of sensation
D. Diminished vision

Explanation: **Explanation:** The cornea is one of the most sensitive tissues in the human body due to its rich nerve supply. The **earliest symptom** of a corneal ulcer is **Pain**. **1. Why Pain is the Correct Answer:** The cornea is densely innervated by the **ciliary nerves** (branches of the Ophthalmic division of the Trigeminal nerve). These nerve endings are located superficially in the corneal epithelium and stroma. Any breach in the epithelium (ulceration) exposes these nerve endings to mechanical irritation from the eyelids and chemical irritation from tears, leading to immediate and severe pain. **2. Analysis of Incorrect Options:** * **Photophobia:** This is a common symptom caused by ciliary body spasm (reflex iridocyclitis) secondary to corneal irritation. While it occurs early, it typically follows the initial onset of pain. * **Loss of sensation:** This is a **sign**, not a symptom. Furthermore, it is characteristic of specific types of ulcers (e.g., Herpes Simplex or Neurotrophic ulcers) rather than being a universal early symptom of all corneal ulcers. * **Diminished vision:** This occurs later as the ulcer progresses to involve the visual axis or as inflammatory debris (hypopyon) and corneal edema increase. **Clinical Pearls for NEET-PG:** * **Cardinal Symptoms:** The classic triad of corneal involvement is **Pain, Photophobia, and Lacrimation**. * **Exception to the Rule:** In **Neurotrophic ulcers** (e.g., post-herpetic or diabetes), pain may be minimal or absent due to damaged nerve endings. * **Fungal vs. Bacterial:** Fungal ulcers often present with less prominent symptoms compared to the severity of the clinical signs (e.g., "silent" presentation with a large hypopyon). * **Deep vs. Superficial:** Pain is often more severe in superficial erosions than in very deep ulcers where nerve endings may have been completely destroyed.

Question 296: Kayser-Fleischer rings in Wilson's disease are seen at the level of which corneal layer?

A. Descemet's membrane (Correct Answer)
B. Epithelium
C. Endothelium
D. Stroma

Explanation: ### Explanation **1. Why Descemet’s Membrane is Correct:** Kayser-Fleischer (KF) rings are the hallmark ocular manifestation of **Wilson’s Disease** (hepatolenticular degeneration). The underlying pathophysiology involves a deficiency in ceruloplasmin, leading to excessive free copper deposition in various tissues. In the eye, copper ions are deposited specifically in the **periphery of Descemet’s membrane**. This occurs because copper ions from the aqueous humor diffuse into the posterior layers of the cornea. **2. Why Other Options are Incorrect:** * **Epithelium:** This layer is involved in conditions like *Hudson-Stahli lines* (iron deposition) or *Vortex keratopathy* (Amiodarone/Fabry’s disease), but not copper deposition in Wilson's. * **Endothelium:** While copper passes through the endothelium to reach Descemet’s membrane, it does not accumulate here. * **Stroma:** Copper deposition in the stroma is seen in *Chalcosis* (intraocular copper foreign body), resulting in a "sunflower cataract," but the KF ring specifically targets the basement membrane (Descemet’s). **3. High-Yield Clinical Pearls for NEET-PG:** * **Appearance:** KF rings appear as golden-brown, ruby-red, or green bilateral rings at the limbus. * **Examination:** They are best visualized using a **Slit-lamp examination** (Gonioscopy can help see the peripheral extent). * **Clinical Significance:** They are present in 95% of patients with neurological Wilson’s and 50-60% of those with hepatic presentation. * **Reversibility:** The KF ring is one of the few corneal deposits that **disappears** with systemic chelation therapy (e.g., D-Penicillamine). * **Differential Diagnosis:** Similar rings can rarely be seen in primary biliary cholangitis or chronic cholestasis due to systemic copper overload.

Question 297: Corneal lesion seen in Hutchinson triad associated with congenital syphilis is:

A. Disciform keratitis
B. Interstitial keratitis (Correct Answer)
C. Phylectenular keratitis
D. Mooren ulcer

Explanation: **Explanation:** **Interstitial Keratitis (IK)** is the correct answer because it is a hallmark ocular manifestation of **late congenital syphilis**. It is an immune-mediated inflammation of the corneal stroma without involvement of the epithelium or endothelium. In syphilis, it typically presents bilaterally and is a key component of **Hutchinson’s Triad**, which consists of: 1. **Interstitial Keratitis** 2. **Hutchinson’s Teeth** (notched incisors) 3. **Sensorineural Deafness** (8th cranial nerve palsy) **Analysis of Incorrect Options:** * **Disciform Keratitis:** This is a deep, circular corneal edema typically caused by a delayed hypersensitivity reaction to the **Herpes Simplex Virus (HSV)**. It is not associated with the Hutchinson triad. * **Phlyctenular Keratitis:** This is a nodular inflammation (allergic response) of the cornea/conjunctiva, most commonly triggered by **Tuberculosis** (endogenous staphylococcal proteins). * **Mooren Ulcer:** This is an idiopathic, peripheral ulcerative keratitis that is painful and progressive, often associated with autoimmune triggers, but unrelated to congenital syphilis. **High-Yield Clinical Pearls for NEET-PG:** * **Salmon Patch:** During the active phase of syphilitic IK, deep vascularization gives the cornea a characteristic pinkish-orange appearance. * **Ghost Vessels:** After the inflammation subsides, the empty, non-perfused vessels remain in the stroma, visible on slit-lamp examination. * **Wessely Ring:** An immune ring seen in the cornea, often associated with disciform keratitis or fungal infections, but distinct from the diffuse stromal involvement of IK.

Question 298: What happens to the corneal endothelium after injury?

A. Slowly regenerates
B. Regenerates rapidly
C. Never regenerates (Correct Answer)
D. Forms a scar

Explanation: **Explanation:** The corneal endothelium is a single layer of hexagonal cells that plays a critical role in maintaining corneal transparency by regulating stromal hydration through an active "pump-leak" mechanism. **1. Why "Never regenerates" is correct:** In humans, corneal endothelial cells are considered **post-mitotic**. They are arrested in the G1 phase of the cell cycle and do not undergo mitosis to replace lost cells. When the endothelium is injured (due to trauma, surgery, or disease), the defect is healed not by cell division, but by the **enlargement (pleomorphism)** and **migration** of existing neighboring cells to cover the bare area. This leads to a progressive decrease in endothelial cell density over time. **2. Why other options are incorrect:** * **A & B (Regeneration):** Unlike the corneal epithelium, which possesses limbal stem cells and regenerates rapidly, the endothelium lacks this regenerative capacity. * **D (Forms a scar):** Scarring (fibrosis) is a feature of the corneal stroma. When the endothelium fails, it leads to **corneal edema** and bullous keratopathy rather than traditional scar tissue formation. **High-Yield Clinical Pearls for NEET-PG:** * **Normal Cell Count:** Approximately 3,000 cells/mm² in young adults. * **Critical Threshold:** If the count falls below **500–700 cells/mm²**, the pump mechanism fails, leading to irreversible corneal edema (Pseudophakic Bullous Keratopathy). * **Specular Microscopy:** The gold standard investigation to assess endothelial cell count and morphology (Polymegathism and Pleomorphism). * **Hassall-Henle bodies:** Physiological hyaline outgrowths of Descemet’s membrane seen in the periphery; when seen centrally, they are called **Guttae** (characteristic of Fuchs’ Endothelial Dystrophy).

Question 299: Vogt's striae are seen in which of the following conditions?

A. Congenital glaucoma
B. Keratoconus (Correct Answer)
C. Aphakia
D. Subluxated lens

Explanation: **Explanation:** **Vogt’s striae** are a classic clinical sign of **Keratoconus**, a non-inflammatory, progressive thinning and cone-like protrusion of the cornea. These are fine, vertical (stress) lines located in the deep posterior stroma and Descemet’s membrane. They are caused by the mechanical stretching of the corneal lamellae due to the ectasia. A key diagnostic feature is that these striae **disappear** temporarily when external pressure is applied to the globe. **Analysis of Options:** * **Keratoconus (Correct):** In addition to Vogt’s striae, other hallmark signs include **Fleischer’s ring** (iron deposition at the base of the cone), **Munson’s sign** (indentation of the lower lid on downgaze), and **Rizutti’s sign**. * **Congenital Glaucoma (Incorrect):** This condition is associated with **Haab’s striae**, which are horizontal or curvilinear breaks in Descemet’s membrane caused by increased intraocular pressure and globe enlargement (buphthalmos). * **Aphakia (Incorrect):** This refers to the absence of the crystalline lens. While it may lead to corneal edema or bullous keratopathy post-surgery, it does not feature Vogt’s striae. * **Subluxated Lens (Incorrect):** This is the partial displacement of the lens (e.g., in Marfan syndrome). It affects refraction and accommodation but does not typically involve structural corneal striae. **High-Yield Clinical Pearls for NEET-PG:** * **Vogt’s Striae:** Vertical lines in Keratoconus (disappear with pressure). * **Haab’s Striae:** Horizontal/Curvilinear lines in Congenital Glaucoma (permanent). * **Hydrops:** An acute complication of Keratoconus caused by a rupture in Descemet’s membrane, leading to sudden corneal edema. * **Treatment Gold Standard:** Rigid Gas Permeable (RGP) contact lenses for vision; Collagen Cross-linking (CXL/C3R) to halt progression.

Question 300: Snow blindness is caused by which type of radiation?

A. Ultraviolet rays (Correct Answer)
B. Infrared rays
C. Microwaves
D. Defect in mirror

Explanation: **Explanation:** **Snow blindness**, clinically known as **Photokeratitis**, is an acute inflammatory condition of the cornea caused by exposure to high-intensity **Ultraviolet (UV) radiation**, specifically **UV-B rays** (wavelength 290–310 nm). Fresh snow reflects nearly 80% of UV radiation, significantly increasing the dose reaching the ocular surface. The corneal epithelium absorbs these rays, leading to protein denaturation and epithelial cell death (desquamation). This results in multiple punctate corneal erosions, causing severe pain, photophobia, and a foreign body sensation, typically occurring 6–12 hours after exposure. **Analysis of Options:** * **A. Ultraviolet rays (Correct):** As explained, UV-B is the primary causative agent for photokeratitis. This is also seen in "Welder’s Flash" (industrial arc welding). * **B. Infrared rays:** These are associated with **Glassblower’s cataract** (True exfoliation of the lens capsule) and thermal burns, but not snow blindness. * **C. Microwaves:** Prolonged exposure to microwaves is linked to **cataractogenesis** due to thermal effects on lens proteins, not acute corneal damage. * **D. Defect in mirror:** This is a distractor and has no clinical relevance to radiation-induced ocular injury. **High-Yield Clinical Pearls for NEET-PG:** * **Management:** Snow blindness is self-limiting. Treatment includes patching the eye, antibiotic ointment to prevent secondary infection, and systemic analgesics. **Topical anesthetics should be avoided** for long-term use as they inhibit epithelial healing. * **Eclipse Retinopathy:** Caused by **UV-A** and visible light (photochemical damage to the fovea). * **Wavelengths:** UV-C (<280 nm) is absorbed by the ozone layer; UV-B (280-315 nm) causes Keratitis; UV-A (315-400 nm) is linked to Cataracts and Macular Degeneration.

Practice by Chapter

Corneal Anatomy and Physiology

Practice Questions

Bacterial Keratitis

Practice Questions

Viral Keratitis

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Fungal Keratitis

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Protozoan Keratitis

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Corneal Degenerations

Practice Questions

Corneal Dystrophies

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Keratoconus and Ectatic Disorders

Practice Questions

Corneal Transplantation

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Corneal Topography and Imaging

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Dry Eye Disease

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Corneal Trauma

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