Mean incubation period for primary syphilitic chancre is
**Core Concept**
The primary syphilitic chancre is a characteristic skin lesion that occurs at the site of infection with Treponema pallidum, the bacterium responsible for syphilis. The incubation period refers to the time elapsed between exposure to the pathogen and the appearance of symptoms.
**Why the Correct Answer is Right**
The mean incubation period for primary syphilitic chancre is approximately 21 days, ranging from 10 to 90 days. This period is influenced by the location of the inoculation site, with lesions often appearing within 2-3 weeks. The chancre typically develops at the site of initial exposure, such as the genitals or oral mucosa, and is caused by the multiplication of T. pallidum bacteria in the skin.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the incubation period for primary syphilitic chancre is not typically less than 10 days, as this would not allow sufficient time for the bacteria to multiply and cause a visible lesion.
**Option B:** This option is incorrect because the incubation period for primary syphilitic chancre is not typically more than 90 days, as this would be an unusually long time for the bacteria to cause a visible lesion.
**Option C:** This option is incorrect because it represents an average incubation period that is not supported by medical literature.
**Clinical Pearl / High-Yield Fact**
The primary syphilitic chancre is a painless, indurated ulcer that may be accompanied by regional lymphadenopathy. It is essential to recognize this characteristic lesion and perform appropriate diagnostic tests to confirm the diagnosis of syphilis.
**Correct Answer: C. 21 days.**