Question

Brandon goes to the doctor to get a physical. The doctor tests Brandon's muscle spindle (stretch) reflex by taping near his patella with a rubber mallet. The doctor reports that the reflex is normal. Two weeks later Brandon was kicked by a steer he was branding. Brandon went back to the doctor who once again taps under Brandon's patella. This time, however, there is no response. Where it the most likely site of Brandon's injury?

a)Ventral posterior thalamus

b)Cerebellum

c)Postcentral gyrus

d)Dorsal root ganglion

e)M1

Answer 1

(D) Dorsal root ganglion is correct answer.

Knee jerk refelx:- It is a sudden kicking movement of the lower leg in response to a sharp tap on the patellar tendon, which lies just below the kneecap.
Knee jerk reflex is type of Strech reflex.
Strech reflex:- The stretch reflex is the contraction of a muscle that occurs in response to its stretch. It is not controlled by higher functioning centre(Like Brain).

As per shown in picture, Dorsal root ganglion is a component of knee jerk reflex arc, if it is damaged there will be no knee jerk reflex.
Thus Brandon most probably injured his dorsal root ganglion.