**Core Concept**
Chvostek's sign is a clinical indicator of hypocalcemia, which occurs when the calcium levels in the blood fall below normal. It is elicited by stimulating the facial nerve, resulting in a twitching of the facial muscles. This phenomenon is related to the decreased neuromuscular transmission caused by low calcium levels.

**Why the Correct Answer is Right**
When Chvostek's sign is elicited, the facial nerve is stimulated by tapping over the parotid gland, causing a contraction of the muscles of facial expression. This occurs due to the release of acetylcholine from the nerve terminals, which binds to nicotinic receptors on the muscle cells. In a state of hypocalcemia, the decreased calcium levels reduce the release of acetylcholine and the excitability of the muscle cells, leading to a twitching response. The facial nerve stimulation in this context is a useful clinical tool for diagnosing hypocalcemia.

**Why Each Wrong Option is Incorrect**
**Option A:** A blood pressure cuff in the arm for 5 minutes is used to elicit Trousseau's sign of latent tetany, not Chvostek's sign. This involves inducing a tetanic contraction of the muscles in the forearm.
**Option C:** Tapping over the extensor pollicis brevis muscle is not a valid method for eliciting Chvostek's sign. This muscle is not involved in the facial nerve's innervation.
**Option D:** Tapping over the flexor retinaculum is not relevant to Chvostek's sign, which involves the facial nerve and the muscles of facial expression.

**Clinical Pearl / High-Yield Fact**
Chvostek's sign is a sensitive indicator of hypocalcemia, but it is not specific. Other conditions, such as hypomagnesemia or hyperthyroidism, can also cause a positive Chvostek's sign. Therefore, it is essential to consider the patient's overall clinical context when interpreting this sign.

**✓ Correct Answer: B. Facial nerve stimulation by tapping over the parotid**