Test 19

1. In the reduced eye:

a. the axial length is 22.50mm

b. the total refractive power of the eye is about +60D

c. nodal point is one at which the visual angle is formed

d. nodal point is at the posterior surface of the lens

e. principal point is within the lens
 

Answer

a.F   b.T   c.T   d.T   e.F
The axial length is 24.13mm. The total refractive 
of the eye is +58.6D. The principal point is anterior to the lens.

2. The following are true about concave mirror:

a. if the object is within the centre of curvature, 
    the image is always enlarged

b. if the object is between the centre of curvature 
    and the point of focus, the image is erect

c. if the object is within the focal point, the image 
    is virtual

d. if the object is outside the centre of curvature, 
    the image is reduced in size

e. if the object is outside the centre of curvature, 
    the image is real.
 

Answer
a. T   b.F   c.T   d.T   e.T
If the object is between the centre of curvature and
the point of focus, the image is inverted, real and enlarged.

3. The following are true about MRI :

a. eye make up should be removed before MRI 

b. possible intraocular foreign body is contraindication for 
    MRI 

c. the patient receives more radiation than CT scan

d. it is superior to CT scan in visualizing bony structures

e. it is the investigation of choice in suspected optic disc 
    drusen

 
Answer
a.F   b.T   c.F   d.F   e.F
MRI makes use of magnetic field and therefore no 
radiation as in CT scan. It visualizes the bone poorly 
compared with CT scan but is superior in the visualization of soft tissues.

4. In applanation tonometry 

a. the area flattened is 13.06mm² 

b. based on Imbert-Fick principle 

c. appropriate area of applanation is achieved when whole 
    tonometer head touches the cornea

d. the intraocular pressure (IOP) can be calculated by the 
    formula area=force/IOP

e. increased corneal curvature is associated with a falsely 
    high IOP
 

Answer
a.F   b.T   c.F   d.T   e.T
The area flattened is 7.35 square mm. Applanation 
tonometry is based on the Inbert-Fick principle, which 
states that for an ideal sphere, the pressure (P) inside the sphere is equal to the force (F) required to applanate (flatten) its surface, divided by the area (A) of flattening:

P = F/A. The area of the tonometer head is larger than the area flattened.

5. The following are true about direct ophthalmoscopy:

a. the field of view is about 25⁰

b. refractive error has a significant effect 

c. the magnification is 15X

d. the field of view increases as the distance between the 
    observer and the patient increases

e. the fundus of a patient with high astigmatism can be 
    sharpened with correcting lenses

Answer
a.F   b.T   c.T   d.F   e.F
The field of view is only 6 degrees. The field of view increases as the distance between the observer and the patient decreases. The fundus of high astigmatism can not be sharpened because the correcting lenses in direct ophthalmoscopy are spherical.

 

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