Meaning of "sort of a knee jerk"

[Koka]

English Language Tests, Intermediate level

ESL/EFL Test #103 "Charlie's Knees", question 5

Then he met Maria as a grown up and fell in love as a sort of knee ......... reaction almost without thinking.

(a) leap
(b) jump
(c) jerk
(d) trip

English Language Tests, Intermediate level

ESL/EFL Test #103 "Charlie's Knees", answer 5

Then he met Maria as a grown up and fell in love as a sort of knee jerk reaction almost without thinking.

Correct answer: (c) jerk

Your answer was: correct
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Hi, what does it mean: "sort of a knee jerk"?
thank you in advance

Koka

[Yankee]

Hi Koka

The idiom is 'knee jerk reaction'. A knee jerk reaction is an 'immediate, automatic response to something'.

There is a good explanation here:
http://www.phrases.org.uk/meanings/220600.html

And there is a further definition here:
http://www.thefreedictionary.com/Knee+jerk+reaction

If someone taps your knee in just the right place, your leg will refexively jerk forward. So, the test sentence is comparing his falling in love with Maria to a knee jerk reaction. It was something that happened immediately -- almost reflexively.

Amy