A patient with HIV has diarrhoea with AF +ve organism in stool. The most likely organism is
Surprised,

Usually, the mycobacterium is associated with respiratory tract infection.
In HIV infections mycobacterium is an important cause of diarrhoeas.
The incidence of the extrapulmonary disease is increased in patients with HIV.

Work up in an HIV patient with diarrhoea.

The initial evaluation of a patient with HIV infection and diarrhoea includes:-
A set of stool examination including culture, examination for ova and parasites. and
Examination for Clostridium difficile toxin.
Approximately 50% of the time this workup will demonstrate infection with pathogenic bacteria, mycobacteria or protozoa.
AFB +ve organism suggests the presence of Mycobacterium.
We have two options to choose from either mycobacterium. T.B. or mycobacterium avium intracellulare.
Both mycobacterium T.B. and M.A.C. can cause diarrhoea in patients with HIV and obviously, both of them are acid-fast, So, which one to choose from.
I am in favour of Mycobacterium avium intracellular.
MAC infections are more likely to cause diarrhoea.
The presumed portals of entry in MAC are respiratory and gastrointestinal tract.
While discussing HIV patients with diarrhoea.  Harrison Says -"Abdominal pain and diarrhoea may be seen with MAC infection."
Thus it seems MAC infection is more likely to cause diarrhoea.
But still, I am not sure of the answer.
The incidence of Mycobacterium T.B. is more than the incidence of MAC in HIV, so mycobacterium T.B. should account for the greater number of diarrhoeal cases in H.I.V.
Mycobacterium avium intracellulare
Mycobacterium tuberculosis
Mycobacterium leprae
Mycoplasmas