What is the most likely diagnosis for a 21-year-old man with severe pain, swelling, and decreased sensation in his lower leg after dropping a weight on it?

The situation described indicates a classic presentation of anterior compartment syndrome. This condition occurs when there is increased pressure within a muscle compartment, typically due to swelling, bleeding, or other factors that compromise circulation and function. In this scenario, the young man's severe pain, swelling, and decreased sensation in the lower leg after a traumatic incident involving a weight are key indicators.

In anterior compartment syndrome, the buildup of pressure can lead to ischemia (lack of blood flow) of the muscles and nerves in the affected compartment, resulting in severe pain that is often disproportionate to the injury, as well as numbness and decreased sensation due to nerve compression. The mechanism of injury—trauma from a weight falling on the leg—supports the likelihood of bleeding or swelling within the compartment.

The other potential diagnoses, while plausible in different contexts, do not align as closely with the acute presentation of severe pain and neurological changes following a compressive injury. A fracture of the tibia might cause pain and swelling, but without the specific symptoms of compartment syndrome. A muscle strain could arise from lifting but would typically present with more localized pain and less severe sensory changes. Deep vein thrombosis, while serious, usually does not present immediately after trauma and would not cause the acute sensory deficits