Meaning of "Decompression sickness manifests"

[Kreta]

English Language Tests, Intermediate level

ESL/EFL Test #172 "Reflexive and intensive pronouns", question 5

Decompression sickness manifests ......... with joint pain and can be fatal.

(a) myself
(b) herself
(c) itself
(d) himself

English Language Tests, Intermediate level

ESL/EFL Test #172 "Reflexive and intensive pronouns", answer 5

Decompression sickness manifests itself with joint pain and can be fatal.

Correct answer: (c) itself
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what means decompression, sickness and manifests. I need some information about that or some explication.

Kreta

[Kitosdad]

Decompression, is the action of coming to the surface after having been deep-diving. Decompression has to be controlled when rising to the surface to allow air in the lungs to normalise according to the depth and rate of ascent. Deep-sea divers often spend a considerable amount of time decompressing on the way back to the surface after a deep dive. Persons who ignore these rules suffer from symptoms which are called " the bends .

Decompression sickness manifests itself with joint pain and can be fatal.
Manifest here means how the person appears and feels. He can vomit and feel severe pains in his joints- It can be fatal, and often is.
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