<prompt> All of the following signs a can be seen in corticospinal tract injury except:Positive Babinski signDifficulty in performing skilled movements of the distal upper limbSuperficial abdominal reflex absentClasp knife spasticity  <response>: Ans. d. Clasp-knife spasticity Ref: Bradleys Neurology in Clinical Practice 5e p2185, 2190Clasp-Knife Effect:When the muscles are hypertonic, as in a UMN lesion, the sequence of moderate stretch - muscle contraction, strong stretch - muscle relaxation is seen.Passive flexion of the elbow meets immediate resistance as a result of the stretch reflex in the triceps muscle.Further stretch activates the inverse stretch reflex. The resistance to flexion suddenly collapses, and the arm flexes.Continued passive flexion stretches the muscle again, and the sequence is repeated.This sequence of resistance followed by a give when a limb is moved passively is known as the clasp-knife effect because of its resemblance to the closing of a pocket knife.It is also known as lengthening reaction because it is the response of a spastic muscle to lengthening. Upper Motor Neuron LesionsLower Neuron LesionsPowerDecreasedQ Weakness Muscle groups or Limbs being affected and not individual musclesQVery much decreased Weakness severe due to paralysis of individual musclesQ.ToneHypertonia Clasp knife spasticityQFlaccidity HypotoniaQWastingAtrophyMinimalQ, if present and due to disuse atrophy.Marked cardinal featureQReflexes Superficial reflexesLostQLostQ Deep reflexesBriskExaggeratedQExaggeratedQ ClonusMay be ntQAbsentQ PlantarExtensorQFlexor or no responseQFasciculationsAbsentQPresentQReaction of degeneration in musclesAbsentQPresentQ 