**Core Concept**
Gynaecomastia and infertility are symptoms associated with an imbalance of sex hormones, specifically an increase in estrogen relative to testosterone. This can be caused by various medications that affect hormone levels or hormone receptors.

**Why the Correct Answer is Right**
Cimetidine, a histamine H2-receptor antagonist used to treat peptic ulcer disease, has anti-androgenic properties. It binds to androgen receptors, thereby inhibiting the action of testosterone and leading to an increase in estrogen levels. This results in gynaecomastia and can also cause infertility by disrupting the normal balance of sex hormones necessary for reproduction. The mechanism of cimetidine involves its ability to cross the blood-brain barrier and interact with androgen receptors in the hypothalamus, leading to a decrease in gonadotropin-releasing hormone (GnRH) secretion and subsequently reduced testosterone production.

**Why Each Wrong Option is Incorrect**
**Option A:** Flutamide is an anti-androgen used to treat prostate cancer, but it does not cause gynaecomastia or infertility. Instead, it blocks the action of testosterone, which is the opposite effect of cimetidine.
**Option C:** Ranitidine is another histamine H2-receptor antagonist similar to cimetidine, but it does not have significant anti-androgenic effects and is not associated with gynaecomastia or infertility.
**Option D:** Methotrexate is an antifolate drug used in chemotherapy and autoimmune disease treatment, and it is not known to cause gynaecomastia or infertility as a side effect.

**Clinical Pearl / High-Yield Fact**
Remember that cimetidine and other histamine H2-receptor antagonists can cause gynaecomastia and infertility due to their anti-androgenic effects. This is a classic example of a medication-induced hormonal imbalance that can have significant clinical consequences.

**✓ Correct Answer: B. Cimetidine**