**Core Concept:** The question is asking about a drug that induces three specific side effects: tachycardia (rapid heart rate), mydriasis (widening of pupils), and urinary retention (difficulty in passing urine). These symptoms are often associated with the actions of some drugs on the autonomic nervous system and their receptors.

**Why the Correct Answer is Right:** The correct answer, **D:** Phenylephrine, is a selective α1-adrenergic receptor agonist. When α1-adrenergic receptors are activated, it leads to:

1. **Tachycardia:** Activation of α1-adrenergic receptors in the sinoatrial (SA) node and atrioventricular (AV) node increases the heart rate and contractility, leading to tachycardia.
2. **Mydriasis:** Activation of α1-adrenergic receptors in the iris causes the pupils to widen, leading to mydriasis.
3. **Urinary retention:** Activation of α1-adrenergic receptors in the prostate and bladder neck results in increased smooth muscle tone, leading to urinary retention.

**Why Each Wrong Option is Incorrect:**

A. **Phenytoin (Option A):** Phenytoin is an anticonvulsant and does not typically cause tachycardia, mydriasis, or urinary retention.
B. **Alprostadil (Option B):** Alprostadil is a prostaglandin E1 analogue used to treat erectile dysfunction and pulmonary hypertension. It does not cause tachycardia, mydriasis, or urinary retention.
C. **Alprazolam (Option C):** Alprazolam is a benzodiazepine used for anxiety and panic disorders. It does not cause tachycardia, mydriasis, or urinary retention.
D. **Phenylephrine (Option D):** As explained above, phenylephrine is a sympathomimetic amine that acts as a selective α1-adrenergic receptor agonist, causing the side effects mentioned.

**Clinical Pearl:** In cases where patients present with symptoms like tachycardia, mydriasis, and urinary retention, clinicians should consider the possibility of phenylephrine administration (or other drugs with similar pharmacological effects). A comprehensive clinical examination should be conducted to identify the cause of these symptoms, which may be due to various conditions like pheochromocytoma, hyperthyroidism, or certain medications like phenylephrine.

**Correct Answer:** D - Phenylephrine is the correct answer as it is a selective α1-adrenergic receptor agonist causing the mentioned side effects.