A 33-year-old man presents to the emergency department with a 3-week history of cough with hemoptysis, fatigue, weight loss. He immigrated from Africa 4 months ago and reports no known past medical history. Gram stain of the sputum is performed and reveals normal oral flora. Chest x-ray reveals cavitary lesions in the upper lobes of each lung.
Which of the following properties of the upper lobes contributes to its seeding by the organism infecting this patient?
D) Increased V/Q ratio in the upper lobes
The patient’s clinical picture is consistent with Mycobacterium tuberculosis infection. The patient comes from an area where tuberculosis is endemic and presents with classic symptoms of cough for 3+ weeks, hemoptysis, fatigue, fever. Additional symptoms commonly seen in tuberculosis include night sweats and weight loss. Knowing that a combination of pulmonary physiology and characteristics of Mycobacterium tuberculosis are key to answering this question. Mycobacterium tuberculosis is a strict aerobe and thus thrives in areas of high oxygen content. The upper lobes of each lung have an increased V/Q ratio (more ventilation than perfusion) and thus excess oxygen compared to the other lobes. Therefore, during secondary activation of tuberculosis, the increased V/Q ratio of the upper lobes are a primary reason for Mycobacterium tuberculosis tropism.
Answer choice A: Decreased immune response in the upper lobes, is incorrect. There is no evidence to suggest this is a valid statement
Answer choice B: Increased perfusion in the upper lobes, is incorrect. There is less perfusion of the upper lobes
Answer choice C: Increased TNF-alpha signaling in the upper lobes, is incorrect. TNF-alpha is secreted by macrophages and is responsible for many of the symptomatic manifestations of tuberculosis.
Answer choice E: Sulfatide glycolipid homing to the upper lobes, is incorrect. Sulfatide glycolipids are used by Mycobacterium tuberculosis to inhibit macrophage lysosomal fusion with the phagosome in which the organism resides.
Key Learning Point
Mycobacterium tuberculosis is the causative agent of tuberculosis. It is often seen in immigrants from endemic areas though may be seen in many other populations as well. The organism prefers the upper lobes of the lungs during secondary reactivation because of the increased V/Q ratio (and thus higher O2) since it is a strict aerobe.
