Final answer:
The most likely diagnosis for a 23-year-old patient with acute dizziness, nausea, vomiting, recent infection, and mild hearing loss is Labyrinthitis, an inner ear disorder causing vertigo and potential hearing impairment (A).
Step-by-step explanation:
Based on the clinical presentation of acute onset of dizziness described as room spinning, accompanied by nausea and vomiting, with a recent history of low-grade fever, myalgias, and a sore throat, the most likely diagnosis is Labyrinthitis.
This condition also aligns with the physical exam findings of mild hearing loss on the left, horizontal nystagmus that resolves with focusing on a fixed object, marked gait instability, and a positive head thrust test, without the presence of dysarthria, motor weakness, sensory loss, facial droop, or limb dysmetria. Labyrinthitis is typically characterized by an inflammatory process affecting the internal ear structures, leading to symptoms of vertigo and often hearing loss.