**Core Concept**
The patient has Turner syndrome, a genetic disorder characterized by the complete or partial absence of one X chromosome, leading to ovarian dysgenesis and primary amenorrhea. The infantile uterus on USG is consistent with ovarian failure and estrogen deficiency.

**Why the Correct Answer is Right**
In Turner syndrome, the absence of one X chromosome results in the non-functional ovaries, which leads to estrogen deficiency and subsequently, the underdevelopment of the uterus. The next step in management is to confirm the diagnosis with karyotyping and to initiate hormone replacement therapy (HRT) to induce puberty and prevent long-term complications such as osteoporosis and cardiovascular disease. HRT typically involves estrogen replacement therapy, which stimulates the growth of the uterus and promotes secondary sexual characteristics.

**Why Each Wrong Option is Incorrect**
* **Option A:** While karyotyping is essential in diagnosing Turner syndrome, it is not the next step in management. Karyotyping has already been done, and the diagnosis is confirmed.
* **Option B:** Hormone replacement therapy is the correct next step, but starting with progesterone alone is not appropriate. Estrogen replacement therapy is required to induce pubertal changes and prevent long-term complications.
* **Option C:** While laparoscopic ovarian drilling may be considered in some cases of primary ovarian insufficiency, it is not the next step in management for a patient with Turner syndrome. This procedure is typically considered for patients with premature ovarian failure due to other causes.
* **Option D:** Genetic counseling is an important aspect of managing Turner syndrome, but it is not the next step in management. The patient has already been diagnosed, and hormone replacement therapy is the priority.

**Clinical Pearl / High-Yield Fact**
Turner syndrome is a genetic disorder that affects 1 in 2,500 female births. It is essential to diagnose and manage Turner syndrome promptly to prevent long-term complications such as short stature, infertility, and cardiovascular disease.

**Correct Answer:** B.