**Core Concept**
Hypoglycemia is a critical condition characterized by abnormally low blood glucose levels. In patients on beta blocker therapy, the typical symptoms of hypoglycemia can be masked due to the blockade of beta-adrenergic receptors, which normally respond to low blood glucose by stimulating the 'fight or flight' response. This response typically manifests as tremors, sweating, tachycardia, and palpitations.

**Why the Correct Answer is Right**
Sweating is a sympathetic nervous system-mediated response that is not directly dependent on beta-adrenergic receptors. The hypothalamus, which regulates body temperature and autonomic responses, is responsible for triggering sweating in response to hypoglycemia. Unlike tremors, tachycardia, and palpitations, which are all mediated by beta-adrenergic receptors, sweating can occur independently of these receptors, making it a more reliable symptom of hypoglycemia in patients on beta blocker therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** Tremor is a classic symptom of hypoglycemia, but beta blockers can effectively mask this symptom by blocking the beta-adrenergic receptors responsible for tremors.

**Option C:** Tachycardia is typically seen in hypoglycemia as a result of increased sympathetic activity. However, beta blockers can prevent the heart rate from increasing in response to hypoglycemia, making tachycardia an unreliable symptom in patients on beta blocker therapy.

**Option D:** Palpitations are another symptom of hypoglycemia that is mediated by beta-adrenergic receptors. Beta blockers can effectively block these receptors, making palpitations an unreliable symptom in patients on beta blocker therapy.

**Clinical Pearl / High-Yield Fact**
In patients on beta blocker therapy, the absence of typical hypoglycemic symptoms such as tremors, tachycardia, and palpitations should prompt clinicians to consider hypoglycemia as a potential diagnosis, and to closely monitor blood glucose levels.

✓ Correct Answer: B. Sweating. Sweating is the only reliable symptom of hypoglycemia in a patient on beta blocker therapy.