A 32-year-old man is brought to the emergency department following a motor vehicle collision with blunt abdominal trauma. On examination, he has left-sided rib fractures (8–10) and left upper quadrant tenderness. Focused assessment with sonography for trauma (FAST) is positive for free fluid in the left upper quadrant. Which organ is most commonly injured in this clinical scenario?

## Most Common Organ Injury in Left Upper Quadrant Blunt Trauma **Key Point:** The spleen is the most frequently injured solid organ in blunt abdominal trauma, accounting for 40–55% of all blunt splenic injuries. Its superficial location in the left upper quadrant, rich vascular supply, and lack of a protective muscular capsule make it highly vulnerable to injury from rib fractures and direct impact. ### Mechanism of Injury The spleen's anatomical position — between ribs 9–11 and protected only by the splenic capsule — means that left-sided rib fractures (especially ribs 8–10) frequently lacerate the splenic parenchyma or rupture the splenic vessels. The positive FAST finding of free fluid in the left upper quadrant is highly suggestive of splenic hemorrhage. ### Clinical Significance **High-Yield:** Splenic injury ranges from Grade I (small laceration) to Grade V (complete shattered spleen). Most Grade I–III injuries can be managed non-operatively with bed rest, ICU monitoring, and serial hemoglobin checks in hemodynamically stable patients. Grade IV–V injuries or hemodynamic instability mandate splenectomy. ### Frequency Comparison | Organ | Frequency in Blunt Abdominal Trauma | Typical Mechanism | |-------|--------------------------------------|-------------------| | Spleen | 40–55% | Rib fractures, left flank impact | | Liver | 35–45% | Right-sided blunt trauma | | Kidney | 10–15% | Flank trauma, especially with pre-existing renal disease | | Pancreas | 3–5% | Crush injury, handlebar injury | **Clinical Pearl:** The spleen is the most commonly injured organ overall in blunt abdominal trauma, but the liver is the most commonly injured organ requiring operative intervention (due to higher grade injuries and hemorrhage). ### Why FAST Matters Here The positive FAST in the left upper quadrant (splenorenal recess) confirms free intraperitoneal fluid, consistent with splenic injury. This finding, combined with left rib fractures and left-sided tenderness, makes splenic injury the most likely diagnosis.