Okay, let's tackle this question. The scenario is a 34-week premature baby with bullous lesions and X-ray showing periostitis. The question is asking for the next investigation.

First, I need to recall the possible causes of bullous lesions in a preterm infant. Common conditions include neonatal bullous impetigo, epidermolysis bullosa, and other infections. But periostitis is mentioned, which is inflammation of the periosteum, the bone's outer layer. That's a key clue.

Periostitis in neonates can be due to infection, trauma, or certain metabolic conditions. However, infections like staphylococcal or streptococcal can cause both skin lesions and periostitis. Another possibility is congenital syphilis. Syphilis in newborns can present with skin lesions, periostitis, and other systemic symptoms.

So the next step would be to investigate for congenital syphilis. The standard test would be a serological test for syphilis, like VDRL or RPR, and then confirm with FTA-ABS. Also, a complete blood count might show anemia or leukopenia, but the question is about the next investigation.

Looking at the options, the correct answer is likely to be a serological test for syphilis. The other options might be things like culture of the skin lesion, which would be for bacterial infection, but without specific findings, serology is more targeted. Alternatively, maybe a bone scan for the periostitis, but the key here is the combination of lesions and periostitis pointing towards syphilis.

**Core Concept**
The presentation of bullous lesions and periostitis in a preterm infant strongly suggests congenital syphilis, a systemic infection caused by *Treponema pallidum*. Serological testing is critical for diagnosis due to the characteristic multisystem involvement in neonatal syphilis.

**Why the Correct Answer is Right**
Congenital syphilis manifests with skin lesions (e.g., snail-track rash, bullae), hepatosplenomegaly, periostitis, and systemic symptoms. Periostitis results from direct treponemal invasion of bone. The next investigation is a **serological test for syphilis** (e.g., VDRL/RPR, followed by FTA-ABS) to confirm maternal-fetal transmission. This is the gold standard over imaging or cultures in this context.

**Why Each Wrong Option is Incorrect**
**Option A:** Skin lesion culture may identify bacterial pathogens like *Staphylococcus aureus* but would miss *Treponema pallidum*, which is unculturable in routine labs.
**Option B:** Bone biopsy is invasive and unnecessary when non-invasive serology is available and highly specific for syphilis.
**Option C:** CBC may show anemia or thrombocytopenia but lacks specificity for syphilis.

**Clinical Pearl / High-Yield Fact**
Remember the "4 Cs" of congenital syphilis: **Congenital** (prematurity), **Cutaneous** (bullous lesions), **Cranial** (hepatosplenomegaly), and **Cranial** (saddle-nose deformity). Always consider syphilis in neonates with un

✓ Correct Answer: A. VDRL for mother & baby