True about H. influenza:
**Core Concept**
_Haemophilus influenzae_ is a Gram-negative coccobacillus that is a common cause of respiratory and invasive infections, particularly in children. It is a capsulated bacterium that requires specific growth factors, such as X and V factors, to grow in vitro.
**Why the Correct Answer is Right**
The correct answer is related to the bacterium's requirement for specific growth factors. _Haemophilus influenzae_ requires X factor, also known as heme, which is necessary for the synthesis of cytochrome c oxidase, and V factor, also known as NAD, which is necessary for the synthesis of FAD and NADPH. The bacterium's inability to synthesize these factors leads to its reliance on external sources. This characteristic is exploited in the laboratory to identify _H. influenzae_.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not describe a characteristic of _H. influenzae_. While _H. influenzae_ is a capsulated bacterium, this fact is not relevant to the question.
**Option B:** This option is incorrect because it is not a specific growth factor required by _H. influenzae_. While _H. influenzae_ does require growth factors, this option does not accurately describe one of them.
**Option C:** This option is incorrect because it does not describe a characteristic of _H. influenzae_. While _H. influenzae_ is a Gram-negative bacterium, this fact is not relevant to the question.
**Clinical Pearl / High-Yield Fact**
_Haemophilus influenzae_ type b (Hib) conjugate vaccines have been widely used to prevent invasive Hib disease, which is a significant cause of morbidity and mortality in children. These vaccines have been highly effective in reducing the incidence of Hib disease.
**Correct Answer:** D.