A 31-year-old woman has been suffering from systemic scleroderma for 14 years. She underwent multiple inpatient treatment courses. She complains of intermittent dull pain in the area of her heart, palpitations, shortness of breath, headache, swollen eyelids, weight loss, and painful and deformed joints in her limbs. The prognosis of her condition will be worse if which of the following organs is affected?