A mother brings her 4-year old daughter Sarah to the emergency department. She explains that at school the previous day, her daughter was reported to trip over a class-mate and fall. The mother explains that they went to a walk-in clinic last night, x-rays were completed and they were sent home in a sling. Her mother feels unsatisfied with the care she received and reports that Sarah is no longer using her left arm and was in pain last night.

General appearance: Overall well looking 4-year old girl, easily distractable, limp left arm 
Vitals: HR: 115 beats/min, BP: 97/66, RR: 22/min, T: 37.0 C 
HEENT: Unremarkable 
CVS: Normal S1, S2, II/VI systolic ejection murmur 
Pulmonary: Chest clear to auscultation 
Abdo: Soft and nontender. 
Neuro: Screening exam normal.

Focused MSK exam:

Upon attempt to closer examine her arm, she becomes anxious and guards her arm. The left arm is in 20 degrees partial flexion and the forearm partially pronated. She is resistant to any movement of the wrist, fingers or arm. There is no pain shrugging the shoulders. There is no erythema, swelling or signs of trauma in any region of the left arm. Neurovascular status is intact.

Based on the history and physical exam, what is the most likely diagnosis?