A 55-year-old man with underlying chronic obstructive pulmonary disease develops a cavitary upper lobe lesion. Colonies grown on Lowenstein-Jensen medium are buff coloured when incubated in the dark but turn bright yellow after brief exposure to light. Which organism fits this description?
- A Mycobacterium avium-intracellulare
- B Mycobacterium scrofulaceum
- C Mycobacterium kansasii ✓
- D Mycobacterium fortuitum
Correct answer: C. Mycobacterium kansasii
Explanation
Photochromogens such as M. kansasii produce carotenoid pigment only after exposure to light, which matches the described colonies. M. scrofulaceum is a scotochromogen, forming yellow-orange pigment even in the dark. M. avium-intracellulare is nonchromogenic, and M. fortuitum is a rapid grower that does not fit the pigment pattern. M. kansasii classically causes cavitary lung disease resembling tuberculosis in patients with pre-existing lung disease.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.