**Core Concept**
Fractured ribs during resuscitation are often caused by chest compressions in cardiopulmonary resuscitation (CPR). The underlying mechanism involves the transmission of force from the compressing fingers to the ribcage, leading to breakage. This phenomenon is more pronounced in certain regions of the ribcage due to anatomical considerations.

**Why the Correct Answer is Right**
The correct answer, C. 4th - 6th ribs, is due to the fact that these ribs are subjected to the greatest force and pressure during chest compressions. The 4th to 6th ribs are more prone to fracture because they are located in the middle of the ribcage, where the force of compression is most concentrated. The ribcage is composed of 24 ribs, with the first seven pairs directly attached to the sternum (breastbone) and the remaining 17 pairs attached to the thoracic vertebrae. During CPR, the force is transmitted through the sternum to the 4th to 6th ribs, making them more susceptible to fracture.

**Why Each Wrong Option is Incorrect**
**Option A:** 2nd - 4th ribs are not the most commonly involved in fractured ribs during resuscitation because they are attached to the sternum and subjected to less force during CPR.
**Option B:** 3rd - 5th ribs are not the most commonly involved because they are located slightly higher in the ribcage and receive less force during chest compressions.
**Option D:** 5th - 7th ribs are not the most commonly involved because they are located lower in the ribcage and receive less force during CPR.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that the risk of fractured ribs during CPR can be mitigated by using a defibrillator with a mechanical chest compression device, such as an LUCAS device, which can reduce the force transmitted to the ribcage.

✓ Correct Answer: C. 4th - 6th ribs