**Question:** Which of the following drug does NOT cause Gynaecomastia?

A. Testosterone
B. Tamoxifen
C. Flutamide
D. Anastrozole

**Core Concept:** Gynaecomastia is the development of enlarged, often painful, breast tissue in males, primarily caused by hormonal imbalances. It can be due to increased estrogen levels or decreased androgen levels.

**Why the Correct Answer is Right:** Anastrozole is an aromatase inhibitor, specifically designed to lower estrogen levels in the body. By inhibiting the enzyme aromatase, it prevents the conversion of androgens to estrogens, thereby reducing the risk of gynaecomastia.

**Why Each Wrong Option is Incorrect:**

A. Testosterone is a naturally occurring hormone responsible for masculinization during puberty and maintaining male characteristics. Testosterone supplementation can lead to gynaecomastia if the androgen-estrogen balance is disrupted.

B. Tamoxifen is a selective estrogen receptor modulator (SERM) that primarily targets estrogen receptors. While it has antiestrogenic effects, it does not target the reduction of estrogen production, which is the key mechanism of Anastrozole.

C. Flutamide is an antiandrogen, inhibiting androgen receptors and decreasing the effects of androgens. However, unlike Anastrozole, it does not inhibit estrogen production, which is crucial for preventing gynaecomastia.

**Clinical Pearl:** A proper understanding of the pharmacology of the drugs involved is crucial when prescribing medications to patients with a history of gynaecomastia or increased breast tenderness. It is essential to balance hormone levels and target the specific mechanisms to prevent or treat gynaecomastia.

**Correct Answer:** D. Anastrozole, as it effectively inhibits aromatase, the enzyme responsible for converting androgens into estrogens, preventing the excess estrogen production that leads to gynaecomastia.