**Core Concept**
The presentation of bullous lesions in a premature newborn, combined with periostitis on X-ray, is suggestive of congenital syphilis. This condition results from the transplacental transmission of Treponema pallidum from an infected mother to her fetus.

**Why the Correct Answer is Right**
The initial investigation for congenital syphilis involves assessing for the presence of Treponema pallidum in the newborn's body. The VDRL (Venereal Disease Research Laboratory) test is a nontreponemal test that detects the presence of antibodies against cardiolipin, a component of the syphilis lipoprotein complex. These antibodies are produced in response to the infection and can be detected in the newborn's serum. The VDRL test is a useful screening tool for congenital syphilis, as it is relatively easy to perform and can provide rapid results.

**Why Each Wrong Option is Incorrect**
**Option A:** The direct fluorescent antibody (DFA) test is a treponemal test that detects the presence of Treponema pallidum antigens in clinical specimens. While it is a specific test for syphilis, it is not the initial investigation for congenital syphilis, as it requires specialized equipment and expertise.

**Option B:** The TPHA (Treponema pallidum hemagglutination assay) test is another treponemal test that detects the presence of antibodies against Treponema pallidum. Like the DFA test, it is not typically used as an initial investigation for congenital syphilis.

**Option C:** The RPR (Rapid Plasma Reagin) test is a nontreponemal test similar to the VDRL test. While it can be used as a screening tool for syphilis, it is not the preferred test for congenital syphilis due to its lower sensitivity and specificity compared to the VDRL test.

**Option D:** The PCR (polymerase chain reaction) test is a molecular test that can detect the presence of Treponema pallidum DNA in clinical specimens. While it is a highly specific test for syphilis, it is not typically used as an initial investigation for congenital syphilis due to its higher cost and complexity.

**Clinical Pearl / High-Yield Fact**
The VDRL test is a useful screening tool for congenital syphilis, but it can produce false-positive results in newborns due to the presence of maternal antibodies. Therefore, a confirmatory test such as the TPHA or DFA test should be performed to confirm the diagnosis.

**Correct Answer:** C. RPR (Rapid Plasma Reagin) test