**Core Concept**
The question involves a preterm baby with bullous lesions and periostitis, suggesting an underlying infection. The clinical presentation is characteristic of congenital syphilis, a condition caused by the bacterium *Treponema pallidum*.

**Why the Correct Answer is Right**
The diagnosis of congenital syphilis in a newborn is typically confirmed by a non-treponemal test, such as the Venereal Disease Research Laboratory (VDRL) test or the Rapid Plasma Reagin (RPR) test. These tests detect the antibodies against the lipoidal material released from damaged host cells. The VDRL test is a flocculation test that measures the level of antibodies against the cardiolipin antigen. A positive result indicates the presence of syphilis, which in this case, is likely congenital.

**Why Each Wrong Option is Incorrect**
**Option A:** A complete blood count (CBC) would not help in diagnosing congenital syphilis. While a CBC can provide general information about the baby's health, it is not specific for diagnosing congenital syphilis.
**Option B:** A lumbar puncture would be useful in diagnosing congenital syphilis if it involves neurological manifestations. However, the question does not mention any neurological symptoms, making it less relevant.
**Option C:** A throat culture for group B streptococcus (GBS) would be useful in diagnosing GBS infection, not congenital syphilis.

**Clinical Pearl / High-Yield Fact**
Congenital syphilis can present with a wide range of symptoms, including bullous lesions, periostitis, and other systemic manifestations. A high index of suspicion is necessary for early diagnosis and treatment to prevent long-term complications.

**Correct Answer:** D. Perform a Venereal Disease Research Laboratory (VDRL) test.