You are answering a USMLE-style medical question. Question: A 4-year-old girl is brought to the emergency department with a persistent cough, fever, and vomiting. The past year the child has been admitted to the hospital 3 times with pneumonia. For the past 1 week, the child has been experiencing thick purulent cough and says that her chest feels ‘heavy’. Her stools have been loose and foul-smelling over the past week. Her parents are also concerned that she has not gained much weight due to her frequent hospital visits. She was born at 39 weeks gestation via spontaneous vaginal delivery and is up to date on all vaccines and is meeting all developmental milestones. On physical exam, the temperature is 39.1°C (102.4°F). She appears lethargic and uncomfortable. Crackles are heard in the lower lung bases, with dullness to percussion. A small nasal polyp is also present on inspection. Which of the following is the most likely cause for the girl’s symptoms? Options: A) Dysfunction in a transmembrane regulator B) Inefficient breakdown of leucine, isoleucine, and valine C) Dysfunction in the motility of respiratory cilia D) Deficiency in lymphocytic activity Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.