**Question:** The mycobacteria which grows in culture with in 1-2 weeks are:

A. Mycobacterium tuberculosis
B. Mycobacterium avium complex (MAC)
C. Mycobacterium kansasii
D. Mycobacterium fortuitum

**Core Concept:**
Mycobacteria are a group of aerobic, rod-shaped bacteria that can cause various diseases in humans and animals. Rapidly growing mycobacteria (RGM) refers to a subset of mycobacteria that grow rapidly within 1-2 weeks in culture media.

**Why the Correct Answer is Right:**
Mycobacterium fortuitum is a rapidly growing mycobacterium that can grow in culture within 1-2 weeks. It is considered a fastidious organism as it requires special culture conditions, including enriched media and temperature control, for optimal growth. This group of mycobacteria is usually found in soil and water, and can cause infections in immunocompromised individuals or as opportunistic infections.

**Why Each Wrong Option is Incorrect:**

A. Mycobacterium tuberculosis is a slowly growing mycobacterium that takes more than 2-8 weeks to grow in culture. It is the primary causative agent of tuberculosis and typically requires specialized media for growth.

B. Mycobacterium avium complex (MAC) consists of several closely related species, including Mycobacterium avium and Mycobacterium intracellulare. These organisms grow slowly and typically require at least 2-8 weeks for detection in culture. While MAC can cause infections, it does not grow within 1-2 weeks.

C. Mycobacterium kansasii is a slowly growing mycobacterium that typically requires 2-8 weeks for detection in culture. It is more commonly found in soil and water and can cause pulmonary and extrapulmonary infections, particularly in immunocompromised individuals.

**Clinical Pearl:**

Rapidly growing mycobacteria (RGM) are a group of mycobacteria that can grow rapidly in culture, usually within 1-2 weeks. They are less common than the well-known tuberculosis causing Mycobacterium tuberculosis and can cause infections in immunocompromised individuals. Diagnosis of RGM infections relies on culture and sensitivity testing, as well as molecular assays like Polymerase Chain Reaction (PCR) for rapid detection.

**Correct Answer Explanation:**

**Correct Answer: D. Mycobacterium fortuitum**

Mycobacterium fortuitum is part of the rapidly growing mycobacteria group and grows rapidly (within 1-2 weeks) in culture media. These organisms can cause infections in immunocompromised individuals or as opportunistic infections in immunocompetent individuals. Diagnosis of M. fortuitum infections often involves culture and sensitivity testing and molecular assays such as PCR for rapid detection.