**Core Concept**
The question tests the understanding of the relationship between excessive exercise and menstrual disorders, specifically amenorrhea. Amenorrhea refers to the absence of menstruation, and it can be classified into two categories: primary (never had a period) and secondary (had a period but stopped). In this case, the patient had a normal menstrual history but developed amenorrhea after starting excessive exercise.

**Why the Correct Answer is Right**
Excessive exercise is a common cause of secondary amenorrhea in athletes, particularly those with a low body mass index (BMI). Prolonged and intense physical activity can disrupt the normal hypothalamic-pituitary-ovarian axis, leading to a decrease in gonadotropin-releasing hormone (GnRH) secretion. This results in low levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which are essential for ovulation and menstruation. The normal USG findings and low-normal LH and FSH levels in this patient support this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Depression can be associated with amenorrhea, but it is not the most likely cause in this case. Depression can lead to hypothalamic amenorrhea through stress-induced GnRH suppression, but the patient's normal BMI and low-normal LH and FSH levels suggest a different etiology.

**Option C:** Premature menopause is not a likely cause of amenorrhea in a 30-year-old woman with a normal BMI and low-normal LH and FSH levels. Premature ovarian failure typically presents with elevated FSH levels and is not associated with excessive exercise.

**Option D:** Anorexia nervosa can cause amenorrhea due to GnRH suppression, but the patient's BMI is within the normal range, making this diagnosis less likely. Anorexia nervosa is typically associated with a significantly low BMI.

**Clinical Pearl / High-Yield Fact**
Athletes with a low BMI are at increased risk of developing amenorrhea due to excessive exercise. It is essential to screen for this condition in female athletes, especially those with a history of intense physical activity.

**✓ Correct Answer: B. Excessive exercise. Excessive exercise is a common cause of secondary amenorrhea in athletes with a low body mass index.**