A 34-year-old woman with an 8-year history of SLE (ANA 1:1280 speckled, anti-dsDNA 480, low C3/C4) presents with 6 months of progressive dyspnoea, pleuritic chest pain, and orthopnoea. Examination reveals shallow rapid respirations, use of accessory muscles, and paradoxical abdominal wall motion when supine. Chest X-ray shows small lung volumes with elevated hemidiaphragms. Pulmonary function testing demonstrates a restrictive pattern (FEV1/FVC 0.85, TLC 54% predicted) with severely reduced maximal inspiratory pressure (MIP 32 cmH2O). Notably, the feature marked **C** in the diagram—FVC falling 32% from upright to supine positioning—is observed. DLCO corrected for lung volume (KCO) is normal. Which of the following best explains the pathophysiology underlying this clinical presentation?