Final answer:
The primary concern for severe pain in the left flank after a tackle in football is spleen injury. The spleen's location just inferior to the diaphragm makes it susceptible to trauma and can lead to referred pain and signs of internal bleeding, such as Kehr's sign.
Step-by-step explanation:
In assessing a 16-year-old boy who was struck in the left flank during a football tackle and is experiencing severe pain, the primary concern is that he may have injured his spleen. The spleen is located in the upper-left quadrant of the abdominopelvic cavity, just inferior to the diaphragm. When the spleen ruptures, blood spills into the abdominal cavity, potentially leading to Kehr's sign, which involves pain referred to the left shoulder, chest, and neck due to pressure on the diaphragm caused by the accumulating hemorrhage. The kidney is another possible site of injury, as it is located in the posterior part of the abdomen, below the diaphragm and adjacent to the spleen. However, given the presentation and common football injury patterns, the spleen is of higher concern. Symptoms of splenic injury include pain in the left flank, potential signs of internal bleeding, and possible referred pain in the left shoulder (Kehr's sign).