A 3-day-old term newborn is examined in the postnatal ward. When the examiner allows the infant's head to drop a few centimetres into the examining hand, the response marked **A** in the diagram is elicited. However, the infant's left arm fails to abduct and extend during the initial phase of this reflex, while the right arm responds normally. The left arm remains adducted and internally rotated. What is the most likely diagnosis?