A 21-year-old female with abdominal pain post-blunt trauma has distended abdomen and referred pain to the shoulder. What is the most likely injury?

In this scenario, the clinical features provided — abdominal pain after blunt trauma, abdominal distension, and referred pain to the shoulder — are suggestive of a splenic injury. The spleen is located in the upper left quadrant of the abdomen, and when it suffers a rupture or significant injury, it can lead to internal bleeding, which contributes to abdominal distension.

Referred pain to the shoulder, also known as Kehr's sign, is particularly associated with splenic injuries. This phenomenon occurs due to the anatomical connections of the diaphragm and the phrenic nerve, which can transmit pain from the diaphragm (often irritated by blood in the abdominal cavity) to the shoulder area.

Injuries to the liver, pancreas, or gallbladder can also cause abdominal pain, but they typically do not present with referred pain to the shoulder in the same manner as a splenic injury. The specific combination of symptoms — including the mechanism of injury, the location of pain, and the presence of abdominal distension — strongly points to the spleen as the most likely injured organ in this case.