Whipple's disease

A 52-year-old man comes to the physician for evaluation of severe pain affecting several joints. The symptoms began 3 months ago. He has also had loose, greasy stools several times daily as well as a 12-lb weight loss. Past medical history is noncontributory. He is not sexually active and does not consume alcohol or illicit substances. Temperature is 37.0°C (98.6°F), pulse is 82/min, respirations are 14/min, and blood pressure is 130/62 mmHg. Cardiac auscultation reveals a soft, high-pitched, early diastolic decrescendo murmur heard best at the left 3rd intercostal space. The left and right knee are warm and tender to palpation, and passive range of motion is limited bilaterally. Abdominal examination reveals intense epigastric pain on palpation. Generalized lymphadenopathy is present. Biopsy of the small intestine reveals periodic acid-Schiff-positive (PAS)-positive macrophages within the lamina propria. Which of the following is the most likely cause of this patient’s underlying condition?