**Core Concept:**
The question is about identifying the most likely organism causing diarrhoea in a patient with HIV, when an AFB (+)ve organism is found in the stool. This scenario involves understanding the common pathogens associated with diarrhoea in HIV patients and their characteristic features. In this case, AFB (+)ve organisms are being looked for, which stand for acid-fast bacilli.

**Why the Correct Answer is Right:**
The correct answer, **C. Mycobacterium tuberculosis**, is the most likely organism in this scenario. Mycobacterium tuberculosis is the causative agent of tuberculosis (TB). In immunocompromised patients like HIV patients, it can cause extra-pulmonary tuberculosis, one of which is diarrhoea. The presence of AFB in the stool is a characteristic feature of TB infection.

**Why Each Wrong Option is Incorrect:**

A. **Mycobacterium avium complex (MAC)** is another mycobacterial infection but is less likely in HIV patients presenting with diarrhoea due to AFB (+)ve stool. MAC is more commonly associated with pulmonary tuberculosis and disseminated infections.

B. **Salmonella species:** While Salmonella can cause enteric fever and diarrhoea, the presence of AFB is not a characteristic feature of Salmonella infections. Salmonella diarrhoea is typically non-bacillary, and AFB (+)ve stool is a clue pointing towards TB.

C. **Kochiella bacilli:** This is an incorrect term referring to Mycobacterium tuberculosis. Kochiella bacilli is a misnomer, and the correct term is Mycobacterium tuberculosis.

D. **Cryptosporidium species:** Cryptosporidiosis can cause watery diarrhoea in HIV patients, but AFB (+)ve stool is not a characteristic feature of Cryptosporidium infections. Cryptosporidium diarrhoea is typically non-bacillary.

**Clinical Pearl:**
In HIV patients presenting with diarrhoea and AFB (+)ve stool, TB should be considered in the differential diagnosis. The presence of AFB in stool can be due to extra-pulmonary TB or disseminated TB, which can weaken the immune system further in HIV patients.

**Core Concept:**
Understanding the characteristics of various pathogens causing diarrhoea is essential for accurate diagnosis and appropriate management in patients with compromised immune systems like HIV patients. AFB (+)ve stool in such patients should trigger a search for TB as one of the differential diagnoses, particularly when other causes of diarrhoea are ruled out.

**Why the Correct Answer is Right:**
Mycobacterium tuberculosis is the causative agent for tuberculosis, and the presence of AFB in stool is a characteristic feature of extra-pulmonary tuberculosis, which can occur in HIV patients as a result of compromised immune system.

**Why Other Options are Incorrect:**

A. Mycobacterium avium complex (MAC) is another mycobacterial infection that can cause diarrhoea in HIV patients, but its