A 25-year-old woman visited her doctor complaining of difficulty breathing during mild exertion. She reported a 7-year history of weakness in her limbs, preventing her from participating in sports during school. Her parents are first cousins and both are healthy, and she has two younger siblings without any medical issues. She denied experiencing muscle cramps or urine discoloration.
During the physical examination, weakness was noted in her shoulder and pelvic girdle muscles, as well as her trunk muscles. When taking deep breaths, her anterior abdominal muscles tended to retract. Tendon reflexes were reduced in all four limbs, but there were no sensory abnormalities.
What is the most probable diagnosis?