**Question:** Which of the following is called as Preisz-Nocard bacillus?

**Core Concept:** Preisz-Nocard bacillus refers to a group of bacteria that cause Nocardiosis, a group of rare but severe infections.

**Why the Correct Answer is Right:** The correct answer is **D.** Nocardia asteroides, also known as Preisz-Nocard bacillus, is a causative agent of Nocardiosis. Nocardiosis is a rare but severe infection caused by aerobic Gram-positive bacteria belonging to the genus Nocardia. These bacteria are saprophytic, meaning they can be found in soil and water, and are opportunistic pathogens.

**Why Each Wrong Option is Incorrect:**

A. **Nocardia farcinica:** This is another species of Nocardia that can cause Nocardiosis. However, it is less pathogenic than N. asteroides and rarely presents as an opportunistic infection in immunocompromised individuals.

B. **Staphylococcus aureus:** This is a Gram-positive cocci bacterium that primarily causes skin and soft tissue infections, not Nocardiosis. Staphylococcus aureus is a different genus and does not belong to the Preisz-Nocard bacillus group.

C. **Mycobacterium tuberculosis:** This is a causative agent of Tuberculosis, a completely different infectious disease compared to Nocardiosis caused by Preisz-Nocard bacillus. Tuberculosis is caused by the bacterium Mycobacterium tuberculosis, whereas Nocardiosis is caused by the Preisz-Nocard bacillus, specifically N. asteroides and N. farcinica.

**Clinical Pearl:** Nocardiosis is an opportunistic infection, often seen in immunocompromised patients, particularly those with HIV/AIDS, organ transplant recipients, and those receiving long-term corticosteroid therapy. Prompt diagnosis and treatment are crucial as these infections can be difficult to treat due to the bacteria's resistance to various antibiotics, including beta-lactams and fluoroquinolones.

**Correct Answer Explanation:**

N. asteroides is part of the Preisz-Nocard bacillus group, which includes other species like N. farcinica, N. nova, and N. transvalense. This group is responsible for causing Nocardiosis, a rare but severe infection that can affect various organ systems, including lungs, skin, and central nervous system.

**Why Preisz-Nocard bacillus is a group:** The Preisz-Nocard bacillus group is named after the two German physicians who first described the infection. Preisz and Nocard identified the first cases of Nocardiosis in the early 20th century. The group includes Nocardia asteroides, Nocardia farcinica, Nocardia nova, and Nocardia transvalense. These bacteria are saprophytic, aerobic, and Gram-positive, rod-shaped bacteria that can cause opportunistic infections in immunocompromised patients.

**Core Concept Overview:**