**Core Concept**
Gynecomastia, characterized by the enlargement of breast tissue in males, is often caused by an imbalance between estrogen and androgen hormones. Estimating hormone levels helps identify the underlying cause, which can be either an excess of estrogen or a deficiency of androgens.

**Why the Correct Answer is Right**
Prolactin is not typically involved in the development of gynecomastia. Prolactin levels are more relevant in conditions such as galactorrhea (spontaneous milk production) or amenorrhea (absence of menstruation) in females. In contrast, gynecomastia is usually associated with an imbalance of sex hormones, primarily estrogen and testosterone.

**Why Each Wrong Option is Incorrect**
**Option A:** Testosterone estimation is crucial in gynecomastia as low testosterone levels can contribute to the development of this condition. This is due to the relative increase in estrogen levels.
**Option C:** Estradiol estimation is also essential in gynecomastia as elevated estradiol levels can cause an imbalance between estrogen and androgen hormones, leading to gynecomastia.
**Option D:** Luteinising hormone (LH) estimation can be useful in gynecomastia as LH stimulates the production of testosterone in the testes. Abnormal LH levels can contribute to hormonal imbalances.

**Clinical Pearl / High-Yield Fact**
When investigating gynecomastia, it's essential to remember the "Rule of 10": 10% of normal males develop gynecomastia, 10% of males with gynecomastia have a serious underlying medical condition, and 10% of these serious conditions are caused by malignancies.

**✓ Correct Answer: B. Prolactin**