**Question:** All are true about streptococcus except:

A. Streptococcus is a Gram-positive coccus.
B. Streptococcus can cause meningitis.
C. Streptococcus is a Gram-negative bacillus.
D. Streptococcus is resistant to penicillin.

**Core Concept:**
Streptococcus is a genus of gram-positive cocci bacteria that can cause various infections in humans. The correct classification of Streptococcus, based on the information provided in the options, is as follows:

* Streptococcus is a Gram-positive coccus, which means it is a small, spherical, and Gram-positive bacterium. This distinguishes it from other bacteria like Gram-negative bacilli mentioned in option C.
* Streptococcus can cause meningitis, which is an infection of the meninges (the protective membranes covering the brain and spinal cord). Meningitis caused by Streptococcus is typically referred to as bacterial meningitis.
* Streptococcus is not mentioned to be resistant or sensitive to penicillin in the options. However, it is important to note that some Streptococcus species can be resistant to penicillin, specifically Streptococcus pneumoniae (pneumococcal pneumonia) and Streptococcus pyogenes (group A streptococcal infections).

**Why the Correct Answer is Right:**

The correct answer ([D]) is right because it addresses the incorrect classification of Streptococcus as a Gram-negative bacillus, which it is not. Streptococcus is a Gram-positive coccus, while Gram-negative bacilli are characterized by their Gram stain results, cell wall structure, and response to various antibiotics.

**Why Each Wrong Option is Incorrect:**

* Option A is incorrect because Streptococcus is not a Gram-negative bacterium. It is Gram-positive, not Gram-negative. Gram-positive bacteria have a thick layer of peptidoglycan in their cell wall and retain the crystal violet dye on Gram staining, while Gram-negative bacteria have a thin layer of peptidoglycan and do not retain the dye, leading to the characteristic "reduced" appearance on Gram stain.
* Option B is incorrect because Streptococcus can cause bacterial meningitis, as mentioned earlier. However, its association with penicillin resistance is not directly addressed in this question. Streptococcus pneumoniae and Streptococcus pyogenes are resistant to penicillin due to the presence of penicillinase enzymes that break down the beta-lactam ring of penicillins, rendering them inactive.
* Option C is incorrect as Streptococcus is not a Gram-negative bacillus. Streptococcus is a Gram-positive bacterium with a thick peptidoglycan layer and a characteristic "coccoid" shape on Gram staining.
* Option D is incorrect because Streptococcus is not mentioned to be resistant or sensitive to penicillin in the question. However, some Streptococcus species can be resistant to penicillin, as explained earlier.

**Clinical Pearl:**

Understanding the correct classification of microorganisms, particularly bacteria, is crucial in clinical practice. Differentiating between