Thoracic X-Ray: Bronchial Obstruction (Uworld Q)

[Silverish]

Just working through Uworld and I've come across an Xray image of a bronchial obstruction looking like: http://bestpractice.bmj.com/best-practice/images/bp/en-gb/1069-11_default.jpg.

I'm just trying to understand why its entirely blanched out like that.
The explanation of Uworld said that with a main-stem bronchial obstruction like this, air already in the alveoli will pass into the blood and soon collapse. You can also see a tracheal deviation towards the side of obstruction here (and in the Uworld Q pic). But if the lung is collapsing, shouldn't that show up as radiolucent finding instead of such a radioopaque finding?

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[IndiaInk567]

The white out is because of mucous and/or bacteria accumulating behind the obstruction. This is not always seen in obstruction (Google it - many examples of clear lungs with obstruction). Key finding is the tracheal deviation toward the side of the lesion for the reasons you write.