Primary TB most commonly involves?

ANSWER: (C) LungREF: Robbins 8th edchapter 8Primary tuberculosis is the form of disease that develops in a previously unexposed, and therefore unsensitized, person.[?] In countries where infected milk has been eliminated, primary tuberculosis almost always begins in the lungs.[?] Typically, the inhaled bacilli implant in the distal airspaces of the lower part of the upper lobe or the upper part of the lower lobe, usually close to the pleura. As sensitization develops, a 1- to 1.5-cm area of gray-white inflammation with consolidation emerges, known as the Ghon focus.[?] Tubercle bacilli, either free or within phagocytes, drain to the regional nodes, which also often caseate. This combination of parenchymal lung lesion and nodal involvement is referred to as the Ghon complex.[?] In approximately 95% of cases, development of cell-mediated immunity controls the infection. Hence, the Ghon complex undergoes progressive fibrosis, often followed by radiologically detectable calcification (Ranke complex), and despite seeding of other organs, no lesions develop.