1. Most cases of A scan appearing in the OSCE are related to the biometry calculation.
    Common questions being:

• the structures represented by each peaks (see diagram below)

a = cornea spike  b = anterior lens spike  c = posterior lens spike  d = retinal spike  e = orbital spike

• the effect of silicone oil on the axial length (the value will be longer than expected as ultrasound travels slower in the silicone oil than vitreous)

• the accuracy of the axial length(s)  ie, if the ultrasound scan is accurate as demonstrated by one retinal peak rather than several peaks; if there is significant differences between the two eyes. Note: the Guideline on Cataract Operation from the Royal College of Ophthalmologists finds that :
- 96% of axial lengths fall within the range 21.0 to 25.5 mm and for 60% 
   this is between 22.5 and 24.5 mm
- in the absence of pathology that might affect eye size (eg, unilateral 
  refractive error, coloboma or staphyloma), most individuals have similar 
  axial lengths in each eye
.
.
2. The following scans may appear in the future examination as the examiners look for new 
    questions. Using the principles of ultrasound can you diagnose or give a differential diagnosis
    for the following scans ? (click on the scans for questions and answers)