**Question:** True about T. pallidum is -

A. It is the causative agent of syphilis
B. It is sensitive to penicillin
C. It is a Gram-positive bacterium
D. It is resistant to most antibiotics

**Core Concept:**
Treponema pallidum is the etiologic agent of syphilis, a sexually transmitted disease. The bacterium belongs to the genus Treponema, which is characterized by its spiral shape and lacks a cell wall. Syphilis is classified into primary, secondary, latent, and tertiary stages depending on the clinical manifestations.

**Why the Correct Answer is Right:**
T. pallidum is indeed the causative agent of syphilis (Option A). Syphilis is caused by the infection with this bacterium, which belongs to the Spirochaetaceae family. The bacterium's spiral shape and lack of a cell wall make it distinctive among bacteria.

**Why Each Wrong Option is Incorrect:**

Option B (penicillin sensitivity) is incorrect. T. pallidum is resistant to most antibiotics, including penicillin. This resistance is due to the bacterium's unique cell wall composition and the presence of penicillinase, an enzyme that inactivates penicillin and other beta-lactam antibiotics.

Option C (Gram-positive staining) is incorrect. Although T. pallidum has some structural similarities to Gram-positive bacteria, it is not considered Gram-positive due to its spiral shape and unique cell wall composition.

Option D (resistance to most antibiotics) is the correct answer, highlighting T. pallidum's resistance to most antibiotics, including penicillin. This resistance is crucial for understanding the treatment challenges in syphilis management.

**Clinical Pearl:**
The resistance of T. pallidum to most antibiotics is a major challenge in treating syphilis. Although penicillin is still considered the first-line treatment for syphilis, the presence of penicillinase enzyme in the bacterium makes it resistant to penicillin. In such cases, alternative treatments like doxycycline, ceftriaxone, or azithromycin may be used for treating syphilis.