What is the most likely diagnosis for a child unable to use her left arm after an incident of pulling?

In a child who is unable to use her left arm after an incident involving pulling, the most likely diagnosis is radial head subluxation, commonly referred to as "nursemaid's elbow." This condition typically occurs when a child's arm is pulled suddenly, which can cause the annular ligament to slip and trap the radial head, resulting in pain and an inability to move the arm properly.

The classic signs of this condition include the child holding the affected arm close to their body with the elbow slightly flexed and a reluctance to use that arm. This presentation aligns well with the scenario described.

Understanding this, other potential diagnoses such as fractures, rotator cuff injuries, and soft tissue injuries may also cause arm pain and limited movement. However, they are less likely in the context of a sudden pulling incident, especially in younger children. Fractures, while possible, would typically result in more pronounced swelling, deformity, or pain with movement, which may not be as subtle as what's observed in nursemaid's elbow. Rotator cuff injuries are far less common in pediatric patients due to their underdeveloped musculature and would generally occur from a different mechanism of injury. Soft tissue injuries, depending on their type and severity, might present