Woman With Edema, Vomiting, and Diarrhea

The patient has had no abdominal or chest pain, hematemesis, hematochezia, melena, jaundice, polyuria, skin infection, upper respiratory tract infection, hematuria, shortness of breath, cough, orthopnea, or fever. She has a 3-pack-year smoking history but has recently quit. The patient's mother had noticed facial puffiness about a year before the onset of her other symptoms. The mother has rheumatoid arthritis. The patient's sister has systemic lupus erythematosus, which was diagnosed at age 17 years. The patient has bilateral grade 4 pitting pedal edema that extends up to the thighs and abdominal wall edema. The neck is supple with no elevated jugular venous pressure or lymphadenopathy. Breath sounds are decreased at the bases on both sides. There are multiple striae on the abdomen but no shifting dullness or fluid thrill. The remainder of the physical findings are unremarkable. Review of laboratory studies performed during the past several months reveals low levels of albumin (0.4 g/dL) and total protein (2.4 g/dL). Results of a urinalysis, 24-hour urinary protein measurement, liver function tests, liver ultrasonography, and transthoracic echocardiography are normal. A pregnancy test is negative. Stool studies are negative for leukocytes, Giardia, and Clostridium difficile toxin. Fecal fat (neutral and split) is normal. The α1-antitrypsin level is elevated to 0.79 mg/g of stool (normal, 0 to 0.62 mg/g of stool). The serum α1-antitrypsin level is 42 mg/dL (normal,83 to 199 mg/dL). Serum protein levels are also below normal. Serum IgG is 195 mg/dL (normal, 636 to 1300 mg/dL); IgA and IgM are within the normal range. Tests for tissue antitransglutaminase antibody, anti-endomysial antibody, and antigliadin antibody are negative. Antinuclear antibody is positive (titer of 1:320); anti–double-stranded DNA and anti-Smith antibodies are negative. Results of a colonoscopy with biopsy and a small-intestine biopsy are normal. Esophagogastroduodenoscopy (EGD) reveals giant, enlarged gastric rugae. Biopsy specimens are shown. What is the most likely cause of this patient's symptoms?