Lenses and Prisms — MCQs
Identify the visual field defect shown in the image.
What is the true statement about retinoscopy with a plane mirror?
What type of refractive error is astigmatism, which is characterized by non-spherical curvature of the cornea or lens?
Which keratometry reading is most accurate in post-LASIK eyes for IOL power calculation?
A radiograph is obtained from a child with scoliosis. What is the name of the angle used to measure spinal curvature?
Treatment of presbyopia is by use of which type of lens?
Maximum correction of myopia can be done by?
1mm change in axial length of the eyeball would change the refracting power of the eye by?
Shortening of 2 mm of axial length of the eyeball causes?
Which instrument is shown below?
Lenses and Prisms Indian Medical PG Practice Questions and MCQs
Question 1: Identify the visual field defect shown in the image.
- A. Binasal hemianopia
- B. Bitemporal hemianopia (Correct Answer)
- C. Homonymous hemianopia
- D. Altitudinal defect
Explanation: ***Bitemporal hemianopia*** - The image shows loss of vision in the **temporal (outer) halves of both visual fields**, which is characteristic of bitemporal hemianopia. - This defect typically results from a lesion at the **optic chiasm**, compressing the crossing nasal retinal fibers, often due to a **pituitary tumor**. *Binasal hemianopia* - This condition involves visual loss in the **nasal (inner) halves of both visual fields**, which is the opposite of what is depicted. - It is a rare defect that can be caused by lesions affecting the **uncrossed temporal retinal fibers** on both sides, such as from bilateral carotid artery aneurysms. *Homonymous hemianopia* - A homonymous hemianopia involves the **same half of the visual field in both eyes** (e.g., right visual field loss in both eyes), resulting from a lesion posterior to the optic chiasm. - The image clearly shows different halves affected in each eye (temporal fields), not the same half. *Altitudinal defect* - An altitudinal defect involves the **loss of vision in the upper or lower half of the visual field** in one or both eyes, respecting the horizontal midline. - The visual field loss shown in the image is vertical, affecting the temporal halves, not the upper or lower halves.
Question 2: What is the true statement about retinoscopy with a plane mirror?
- A. In myopia, the red glow moves in the same direction.
- B. Retinoscopy is done at 1 meter away from the patient. (Correct Answer)
- C. In hypermetropia, the red glow moves in the opposite direction.
- D. In emmetropia, the red glow moves in the opposite direction.
Explanation: ***Retinoscopy is done at 1 meter away from the patient.*** - Retinoscopy is typically performed at a **working distance** of 67 cm or 1 meter, to allow for the examiner to observe the reflex and to incorporate a working distance lens in the final calculation. - A 1-meter working distance requires a **-1.00 D sphere correction** to be subtracted from the spherical power found in retinoscopy to find the patient's actual refractive error. *In myopia, the red glow moves in the same direction.* - In **myopia**, using a plane mirror, the retinal reflex appears to move in the **opposite direction** to the movement of the retinoscope. - This "against" movement needs **concave (minus)** lenses to neutralize it. *In hypermetropia, the red glow moves in the opposite direction.* - In **hypermetropia**, using a plane mirror, the retinal reflex appears to move in the **same direction** as the movement of the retinoscope. - This "with" movement needs **convex (plus)** lenses to neutralize it. *In emmetropia, the red glow moves in the opposite direction.* - In **emmetropia**, the light from the retinoscope is focused on the retina, and the reflex also moves in the **same direction** as the retinoscope (when using a plane mirror) until neutralization. - An **emmetropic eye** theoretically requires no corrective lens, other than the working distance correction, to neutralize the reflex.
Question 3: What type of refractive error is astigmatism, which is characterized by non-spherical curvature of the cornea or lens?
- A. Spherical aberration
- B. Curvatural ametropia (Correct Answer)
- C. Index ametropia
- D. Axial ametropia
Explanation: ***Curvatural ametropia*** - Astigmatism, due to its **irregular corneal or lenticular curvature**, falls under the category of curvatural ametropia. - This type of ametropia occurs when the **optical power of the eye varies in different meridians**, leading to light focusing at multiple points rather than a single focal point. *Spherical aberration* - **Spherical aberration** is an optical error where light rays passing through the periphery of a lens focus at a different point than those passing through the center. - It results in a **loss of image clarity** but is distinct from astigmatism's power variation across meridians. *Axial ametropia* - **Axial ametropia** refers to refractive errors caused by an abnormal **length of the eyeball** (either too long or too short). - **Myopia** and **hyperopia** are primary examples of axial ametropia, where the eyeball length dictates whether light focuses in front of or behind the retina, respectively. *Index ametropia* - **Index ametropia** arises from variations in the **refractive index of the ocular media**, such as the cornea, lens, or vitreous humor. - Changes in the refractive index can alter how light bends, but astigmatism is primarily due to surface curvature, not changes in media refractive index.
Question 4: Which keratometry reading is most accurate in post-LASIK eyes for IOL power calculation?
- A. Topography-derived K
- B. Total corneal power (Correct Answer)
- C. Manual keratometry
- D. Automated keratometry
Explanation: ***Total corneal power*** * After LASIK, the **anterior and posterior corneal curvatures** are altered, leading to discrepancies in standard keratometry readings. * **Total corneal power** methods, such as those derived from **corneal tomography** or **anterior segment OCT**, account for both surfaces, providing a more accurate estimation of the true refractive power. * *Topography-derived K* * While corneal topography is valuable for assessing the anterior corneal surface and identifying **irregular astigmatism**, it traditionally focuses on the anterior curvature and may not fully account for the altered **posterior corneal surface** after LASIK. * Standard topography-derived K values often rely on assumptions about the posterior-to-anterior corneal curvature ratio, which are invalid after refractive surgery. * *Manual keratometry* * Manual keratometry measures the **anterior corneal curvature** at a few discrete points and is highly susceptible to inaccuracies due to the post-LASIK changes in corneal shape. * It tends to **overestimate the corneal power** in eyes that have undergone myopic LASIK and **underestimate it** in hyperopic LASIK, leading to significant IOL power calculation errors. * *Automated keratometry* * Similar to manual keratometry, automated keratometry primarily measures the **anterior corneal surface** and relies on a fixed refractive index ratio that is no longer valid after corneal reshaping. * These devices generally provide **inaccurate keratometry readings** in post-refractive surgery eyes, contributing to refractive surprises after cataract surgery.
Question 5: A radiograph is obtained from a child with scoliosis. What is the name of the angle used to measure spinal curvature?
- A. Bohler's Angle
- B. Ferguson's Angle
- C. Cobb's Angle (Correct Answer)
- D. Pauwels' Angle
Explanation: **Cobb's Angle** - **Cobb's angle** is the primary method for measuring the severity of **scoliosis** on radiographs. - It is measured by drawing lines parallel to the superior endplate of the most tilted superior vertebra and the inferior endplate of the most tilted inferior vertebra of the curve; the angle between these two lines (or their perpendiculars) is the Cobb angle. *Bohler's Angle* - **Bohler's angle** is used in the assessment of **calcaneus fractures** and is measured on a lateral foot radiograph. - A decrease in this angle is indicative of a calcaneal fracture. *Ferguson's Angle* - **Ferguson's angle**, also known as the lumbosacral angle, measures the inclination of the sacrum relative to the horizontal in the standing position. - It is primarily used in the assessment of **spondylolisthesis** and other lumbosacral conditions. *Pauwels' Angle* - **Pauwels' angle** is used to classify **femoral neck fractures** based on the angle of the fracture line relative to the horizontal. - It helps determine the severity and stability of femoral neck fractures, guiding treatment decisions.
Question 6: Treatment of presbyopia is by use of which type of lens?
- A. Convex (Correct Answer)
- B. Concave
- C. Biconcave
- D. Concavoconvex
Explanation: **Convex** - **Presbyopia** is an age-related condition where the **lens stiffens**, impairing its ability to accommodate and focus on near objects. - **Convex lenses** add converging power to the eye, helping to bring near objects into focus on the retina. *Concave* - **Concave lenses** diverge light rays and are used to correct **myopia (nearsightedness)**, where the eye focuses images in front of the retina. - They spread light out before it enters the eye, pushing the focal point back onto the retina. *Biconcave* - **Biconcave lenses** are a type of concave lens with two concave surfaces, used for correcting severe **myopia (nearsightedness)**. - These lenses further diverge light rays and are not suitable for presbyopia, which requires converging power. *Concavoconvex* - A **concavoconvex lens** has one concave and one convex surface; its overall power depends on the relative curvatures of the two surfaces. - While some forms might be used in specialized optical systems, they are not the primary or standard correction for presbyopia, which typically requires a simple converging (convex) power.
Question 7: Maximum correction of myopia can be done by?
- A. Radial keratotomy
- B. LASIK (Correct Answer)
- C. Photorefractive keratectomy
- D. Orthokeratology
Explanation: ***LASIK*** - **LASIK (Laser-Assisted In Situ Keratomileusis)** allows for significant correction of high myopia by reshaping the cornea with an excimer laser. - It involves creating a **corneal flap** and then ablating tissue underneath, offering precise and stable vision correction for a wide range of refractive errors. - Among the given corneal refractive procedures, LASIK can correct myopia up to **-10 to -12 D**. *Radial keratotomy* - **Radial keratotomy (RK)** involves making radial incisions in the cornea to flatten it, primarily used for low to moderate myopia (up to -3 to -4 D). - It has a higher risk of **unpredictable outcomes**, induced astigmatism, and glare compared to modern laser procedures. *Photorefractive keratectomy* - **Photorefractive keratectomy (PRK)** involves direct ablation of the corneal surface without creating a flap, which is suitable for moderate myopia (up to -8 to -10 D). - While effective, PRK typically has a **longer recovery period** and more post-operative pain than LASIK. *Orthokeratology* - **Orthokeratology (Ortho-K)** uses specially designed rigid contact lenses worn overnight to temporarily reshape the cornea and correct myopia. - The effect is **temporary**, requiring continuous lens wear to maintain vision correction, and is generally limited to low to moderate myopia (up to -4 to -6 D).
Question 8: 1mm change in axial length of the eyeball would change the refracting power of the eye by?
- A. 1D
- B. 2D
- C. 3D (Correct Answer)
- D. 4D
Explanation: ***3D*** - A 1mm change in the **axial length** of the eyeball leads to an approximate **3 diopter (D) change** in the refractive power of the eye. - This relationship is crucial for understanding **refractive errors** like myopia (if the eyeball is too long) or hyperopia (if it's too short). *1D* - A 1D change in refractive power corresponds to a much larger change in the **focal length** of the eye, not typically 1mm in axial length. - This value is too small to reflect the significant impact of a 1mm axial length alteration on the eye's focusing ability. *2D* - While a direct relationship exists, 2D is an **underestimation** of the actual refractive change caused by a 1mm alteration in axial length. - This value would imply a less sensitive optical system than the human eye. *4D* - A 4D change would represent an **overestimation** of the refractive power change for a 1mm alteration in axial length. - Such a high value is generally seen with more substantial anatomical variations or surgical interventions.
Question 9: Shortening of 2 mm of axial length of the eyeball causes?
- A. 3D myopia
- B. 2D myopia
- C. 6D hypermetropia (Correct Answer)
- D. 1D hypermetropia
Explanation: ***6D hypermetropia*** - A 1 mm shortening of the **axial length** of the eyeball typically results in approximately **3 diopters** of hypermetropia. - Therefore, a 2 mm shortening would cause **6 diopters** (2 mm x 3 D/mm) of hypermetropia. *3D myopia* - Myopia (nearsightedness) is caused by an **eyeball that is too long** or a cornea that is too steeply curved, not by a shortened axial length. - A 2 mm shortening would cause **hypermetropia** (farsightedness), not myopia. *2D myopia* - This option incorrectly identifies both the **type of refractive error** (myopia instead of hypermetropia) and the magnitude of the change. - Shortening of the axial length makes the eye effectively **farsighted**, not nearsighted. *1D hypermetropia* - While reflecting the correct type of refractive error (hypermetropia), the **magnitude is incorrect**. - A 1 mm change in axial length results in about 3 diopters, so 2 mm would be **6 diopters**, not 1 diopter.
Question 10: Which instrument is shown below?
- A. Retinoscope
- B. Direct ophthalmoscope
- C. Indirect ophthalmoscope
- D. Slit lamp biomicroscope (Correct Answer)
Explanation: ***Slit lamp biomicroscope*** - The image clearly depicts a **slit lamp biomicroscope**, characterized by its high-magnification binocular microscope and a slit illuminator - This instrument is used for detailed examination of the **anterior segment structures** of the eye, such as the cornea, iris, and lens - The characteristic features include a **chin rest**, **forehead rest**, and **joystick control** for precise positioning *Retinoscope* - A **retinoscope** is a handheld instrument used to objectively determine the **refractive error** of an eye through observation of the reflective properties of light from the retina - It does not resemble the large, mounted device with a chin rest shown in the image *Direct ophthalmoscope* - A **direct ophthalmoscope** is a handheld device used to view the **posterior segment** of the eye, particularly the retina and optic disc, directly through the pupil - It is much smaller and does not have the complex mechanical stage and chin rest seen in the image *Indirect ophthalmoscope* - An **indirect ophthalmoscope** is typically worn on the examiner's head and used in conjunction with a **condensing lens** to provide a wider, stereoscopic view of the retina - Its appearance is distinctly different from the instrument in the image, which is a stationary examination unit
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