**Core Concept**
The patient's symptoms are characteristic of benign paroxysmal positional vertigo (BPPV), a condition caused by the movement of small crystals (otoconia) within the semicircular canals of the inner ear. This movement stimulates the hair cells, leading to abnormal vestibular signals and vertigo.

**Why the Correct Answer is Right**
The Dix-Hallpike maneuver, which reproduces the patient's symptoms, is a diagnostic test for BPPV. When the patient is positioned in a specific way, the otoconia in the posterior semicircular canal are displaced into the canal, causing vertigo. The patient's symptoms, including the "spinning" sensation and brief duration, are consistent with BPPV. The lack of other neurologic symptoms and normal physical examination also support this diagnosis.

**Why Each Wrong Option is Incorrect**

**Option A:** Basilar migraine can cause vertigo, but it is typically accompanied by other symptoms such as headache, nausea, and visual disturbances. The patient's symptoms are too localized and brief to suggest a migrainous cause.

**Option B:** Brain stem ischemic events can cause vertigo, but they are typically associated with other neurologic symptoms such as weakness, numbness, or difficulty speaking. The patient's normal physical examination and lack of other symptoms make this diagnosis unlikely.

**Option C:** Benign cerebellar tumors can cause vertigo, but they would typically be associated with other symptoms such as ataxia, coordination problems, or hearing loss. The patient's normal physical examination and lack of other symptoms make this diagnosis unlikely.

**Clinical Pearl / High-Yield Fact**
BPPV is a common cause of vertigo in older adults, and the Dix-Hallpike maneuver is a simple and effective diagnostic tool. Remember that BPPV is often associated with aging, head trauma, or inner ear infections, and treatment typically involves canalith repositioning maneuvers or vestibular rehabilitation therapy.

**✓ Correct Answer: D. calcium debris (calcium carbonate crystals) in the semicircular canals**