Tuberculosis is a chronic, communicable disease in which the lungs are the prime target. The disease is caused principally by Mycobacterium tuberculosis hominis (Koch bacillus), but infection with other species occurs, notably M. tuberculosis bovis (bovine tuberculosis). Primary tuberculosis consists of lesions in the lower lobes and subpleural space, referred to as the Ghon focus. The infection then drains to hilar lymph nodes. The combination of Ghon focus and hilar lymphadenopathy is known as "Ghon complex." The typical lesion of tuberculosis is a caseous granuloma, with a soft core surrounded by epithelioid macrophages, Langhans giant cells, lymphocytes, and peripheral fibrosis. Noncaseating granulomas (choice D) are a feature of sarcoidosis, among other causes.Diagnosis: Primary tuberculosis