## **Core Concept**
Otitis externa haemorrhagica, also known as malignant otitis externa, is a severe infection of the external auditory canal and temporal bone. It is primarily caused by bacterial infections, most commonly by *Pseudomonas aeruginosa*. This condition is often seen in diabetic or immunocompromised patients.

## **Why the Correct Answer is Right**
The correct answer, *, is right because *Pseudomonas aeruginosa* is a gram-negative rod that is known to cause otitis externa haemorrhagica, particularly in vulnerable populations such as diabetics or those with compromised immune systems. This bacterium thrives in moist environments and can cause severe, necrotizing infections.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while other bacteria can cause otitis externa, the specific term "otitis externa haemorrhagica" or malignant otitis externa is most closely associated with *Pseudomonas aeruginosa*.
* **Option B:** This option is incorrect as it does not specify a known pathogen associated with otitis externa haemorrhagica.
* **Option C:** This option is incorrect because it also does not specify a known pathogen associated with otitis externa haemorrhagica.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that malignant otitis externa is a serious condition that requires prompt treatment, often with antibiotics such as ciprofloxacin, and sometimes surgical debridement. It is crucial to consider this diagnosis in diabetic patients presenting with severe ear pain and otorrhea.

## **Correct Answer:** D. Pseudomonas aeruginosa.