I. Radiological Findings

From the provided CT images and external photographs, there is a relatively large, soft mass located in the sacrococcygeal region. The overlying skin appears intact, showing no obvious ulceration or discharge. The axial CT view indicates that this mass is situated subcutaneously in the sacrococcygeal area and shows relatively well-defined boundaries. Compared to the surrounding tissues, the lesion primarily demonstrates soft tissue density without apparent calcification or fatty density shadows. It protrudes posteriorly towards the sacrococcygeal region.

Based on the current imaging data, the lesion is mostly seen on the external side of the sacrococcygeal area without significant evidence of extension into the spinal canal. No obvious bony destruction or deformity is noted, and the pelvic organs appear to be in normal positions. Overall, the findings are suggestive of a congenital sacrococcygeal mass, consistent with common features of a congenital sacrococcygeal teratoma.

II. Potential Diagnoses

1. Sacrococcygeal Teratoma (SCT)
   Reason: This is the most common solid tumor in the neonatal period, typically presenting as a congenital mass in the sacrococcygeal region, with a higher incidence in females (approximately 4:1). On imaging, the lesion often appears as a mixed cystic and solid mass. Serum AFP (alpha-fetoprotein) may be elevated in malignant or diverse component cases.
2. Myelomeningocele or Lipoma-like Lesion
   Reason: Congenital neural tube defects or lipomas sometimes present as sacrococcygeal masses. Imaging may reveal connections to the spinal cord or intrathecal space and associated neural structure changes.
3. Other Rare Sacrococcygeal Tumors or Cysts
   Reason: Examples include very rare congenital encephaloceles (typically involving the cranial/spinal region) or cysts near the spinal column. However, these entities are less commonly encountered compared to teratomas based on clinical and imaging findings.

III. Most Likely Final Diagnosis

Considering the patient's sex, the presence of a congenital mass, clinical palpation, and imaging characteristics, the most likely diagnosis is Sacrococcygeal Teratoma. Further evaluation of tissue components and benign or malignant properties would require additional assessments, such as serum alpha-fetoprotein (AFP) measurement and histopathological examination.

IV. Treatment Plan and Rehabilitation

Treatment Strategy:

• For benign teratomas, surgical resection including the coccyx is typically required to reduce the risk of recurrence.
• If intraoperative or postoperative pathology indicates malignant components, adjuvant radiotherapy or chemotherapy may be needed, followed by surgery or additional treatment as appropriate.

Rehabilitation and Exercise Prescription Suggestions:

Given the patient’s young age, postoperative rehabilitation should be individualized and implemented gradually. The following serve as example guidelines:

1. Early Postoperative Stage (1–2 weeks post-surgery)
   • Focus on wound care and basic activity training, avoiding excessive traction on the sacrococcygeal incision.
   • Perform simple passive movements of the lower limbs, such as gentle flexion and extension of the ankles and knees, to promote blood circulation.
2. Intermediate Postoperative Stage (3–6 weeks post-surgery)
   • With a professional rehab therapist or under parental guidance, gradually introduce core and lower-limb muscle-strengthening exercises (e.g., bridging, side-lying leg raises), while carefully monitoring range of motion and intensity.
   • Avoid prolonged sitting or direct pressure on the sacrococcygeal area. The use of soft cushions or specialized seating pads may be beneficial.
3. Late Postoperative Stage (6 weeks and beyond)
   • Under the supervision of a physician or therapist, moderate running, jumping, and other activities may be introduced, but only if the surgical site is fully healed and free of pain or discomfort.
   • Consider age-appropriate, play-based training (e.g., basic balance exercises, low-intensity stretching) to maintain the child’s motivation and improve overall coordination.

During the entire rehabilitation process, close attention should be paid to incision healing and any signs of swelling, pain, or recurrence in the sacrococcygeal region. If any discomfort or suspected recurrence occurs, seek medical consultation promptly.