Which organism is the most common photochromogenic mycobacterium isolated in the United States?

• A. M. bovis
• B. M. kansasii
• C. M. tuberculosis
• D. M. xenopi

Answer: (B)

Photochromogenic mycobacteria are those that produce pigment after exposure to light, a trait used to classify them in the Runyon system. Among these light-activated pigment producers, the one most frequently isolated in the United States is Mycobacterium kansasii. It commonly causes a TB‑like pulmonary illness and is environmental, often linked to water sources, which helps explain why it appears most often in clinical isolates when focusing on this pigment group. The other organisms listed are not photochromogenic—Mycobacterium tuberculosis and Mycobacterium bovis are nonpigmented, and Mycobacterium xenopi is typically scotochromogenic (pigment forms in the dark), not photochromogenic.