This patient has a pneumothorax. Hyperventillation is not enough to compensate for the overall decrease in lung surface area.

x-ray corresponds to a tension pneumothorax = imminent respiratory failure if untreated. Right lung is fully collapsed, increasing intra-thoracic pressure, imparing O2 exchange (due to mass effect toward left lung, and collapsed right one), hence accumulating CO2 (in blood), inducing respiratory acidosis.

https://www.kaggle.com/c/siim-acr-pneumothorax-segmentation/discussion/98498

exact picture from the exam explained. :)

I thought of this as a giant physiologic shunt (ie, due to the pneumothorax there is no ventilation to an entire lung, as a consequence you retain CO2) - not sure if this is the actual mechanism but it helped me get this question right

hopefully this helps!

Keycompany gave the quick, simple explanation.

but from a less-clinical perspective: If anyone has ever been "Lit-up" on the football field, or just generally ever had the "wind-knocked out" of them, you know that your breathing for the next 5 minutes is very shallow because it just hurts to breath too deep.

This girl broke a rib so likely can't breath very deep, so even though she is breathing rapidly the CO2 is likely remaining in her lungs and causing a respiratory acidosis.

Also, here is why the other answers don't make sense:

Amyloidosis & carcinoid - long term problems not related to a broken rib and acute presentation Empyema- "collection of pus" wouldn't form this soon and if it did the percussion would not reveal increased tympany. Pulmonary edema- percussion would not reveal increased tympany. PE- Could cause similar symptoms but percussion would not reveal increased tympany, and the x-ray and history of trauma tells you that this is definitely a pneumothorax.