Question

A 68-year-old man comes to the clinic reporting a 2-month history of worsening fatigue, shortness of breath when walking up stairs, and a "tingling" sensation in his toes. He has a history of hypothyroidism, chronic obstructive pulmonary disease (COPD), and major depressive disorder. His medications include levothyroxine, inhaled tiotropium, and sertraline.

Vital signs are temperature $37.1^\circ\text{C}$ ($98.8^\circ\text{F}$), heart rate $98\text{/min}$, blood pressure $130/80\text{ mm Hg}$, respiratory rate $16\text{/min}$, and oxygen saturation $96\%$ on room air. Physical examination reveals pale conjunctivae and a smooth, glossy, red tongue. Neurologic examination demonstrates diminished vibratory sensation in the bilateral lower extremities and a broad-based gait. Lungs are clear to auscultation bilaterally.

Laboratory evaluation shows:
Hemoglobin: $7.4\text{ g/dL}$
Mean corpuscular volume (MCV): $116\text{ fL}$
Thyroid-stimulating hormone (TSH): $2.8\text{ \mu U/mL}$ (normal: $0.5 - 5.0\text{ \mu U/mL}$)

An arterial blood gas analysis on room air shows a pH of $7.40$ and $\text{PaCO}_2$ of $40\text{ mm Hg}$.

Which of the following is the most likely diagnosis explaining this patient's current presentation?