hypoxia perfusion ventilation mismatch dead space

Uworld says this is v/q mismatch but that gets me confused cause why wouldn't a vent defect give v/q mismatch. That's why you will see a drop in PaO2 in PE(minor). So this one is tougher but: Let's assume you have a well ventilated alveoli, when there is poor oxygen content, vasculature will constrict and redistribute this blood to other alveolar however it will overwhelm the capacity of extracting oxygen into blood - so adding oxygen will help.

Dead space/physiologic dead space/V/Q mismatch to inifinty.

I remember total pulm shunt as if pulm art partial pressure = pulm vein partial pressure, shunted, basically just bypassed lungs. The classical gas exchange theory assumes that the dead space has a sharp, all-or-nothing transition boundary as if it is formed only by the ventilation. These alveolar are already extracting maximum oxygen, adding more would not help.

Lets assume this kid have some alveoli that are working. pulm shunting - ventilation defect V/Q 0, perfusion intact.So this is my understanding of the topic and I might be wrong so please correct me if thats the case:

Hypoxia perfusion ventilation mismatch dead space