Step-by-Step Calculation

Given

• Barometric pressure (PB) = 275 mmHg (~27,000 ft altitude, ~1/3 of sea-level 760 mmHg)
• Patient is breathing atmospheric air (FiO₂ = 21%)

Step 1: Inspired PO₂ (PiO₂)

PiO₂ = 0.21 × (275 - 47) = 0.21 × 228 = 47.9 mmHg

Step 2: Alveolar PO₂ (PAO₂) using the Alveolar Gas Equation

PAO₂ = 47.9 - (24 / 0.8) = 47.9 - 30 = ~18-28 mmHg

Step 3: Oxygen Saturation from the O₂-Hemoglobin Dissociation Curve

Answer: 40% (Option B)

Guyton and Hall Textbook of Medical Physiology, Table 44.1 - Effects of Acute Exposure to Low Atmospheric Pressures on Alveolar Gas Concentrations and Arterial Oxygen Saturation

Answer: Internal Auditory Meatus (Cerebellopontine Angle)

Why This Combination Localizes Here

The Logic:

1. Sensorineural hearing loss = lesion of CN VIII (vestibulocochlear nerve) peripheral to the brainstem cochlear nuclei. Unilateral SNHL must arise outside the brainstem, because cochlear nucleus projections are bilateral centrally.
2. Facial sensation loss = lesion of CN V (trigeminal nerve).
3. Both CN V and CN VIII (along with CN VII - facial nerve) pass through/near the internal auditory meatus and the cerebellopontine angle (CPA). This is the only site where a single lesion can simultaneously damage both.

Classic CPA Lesion Syndrome:

• SNHL + tinnitus (CN VIII)
• Facial sensation loss / decreased corneal reflex (CN V)
• Facial weakness (CN VII)

Why the Other Options Are Wrong:

• Pons - A pontine lesion would produce contralateral body signs + ipsilateral CN palsies, and unilateral SNHL does not arise from brainstem due to bilateral ascending pathways
• Medulla - Houses CN IX-XII nuclei, not CN V or VIII
• Cortex - Cortical lesions do not cause peripheral-type SNHL; auditory cortex receives bilateral input

Neuroanatomy through Clinical Cases, 3rd Ed.: "Since unilateral hearing loss must be caused by a lesion outside the brainstem, the most parsimonious explanation is a lesion in the cerebellopontine angle, where CN V, VII, and VIII all lie in close proximity."

Scott-Brown's Otorhinolaryngology: "Lesions of the CPA cause... sensorineural hearing loss, tinnitus or vertigo... other cranial nerves may also be affected resulting in altered facial sensation."

Answer: C - Small Intestinal Biopsy

The Diagnostic Framework

Serology vs. Biopsy

Why Biopsy Remains the Gold Standard

1. False-positive serology occurs - Conditions like congestive heart failure, chronic liver disease, recent enteric infection, and hypergammaglobulinemia can all cause mildly elevated celiac antibody titers via cross-reacting antibodies
2. "Potential celiac disease" exists - Patients may produce tTG/DGP antibodies with NO intestinal damage; biopsy alone distinguishes true celiac disease from this state
3. Life-long treatment stakes - A gluten-free diet is burdensome for life; a false-positive diagnosis from serology alone is unacceptable
4. Serology can be falsely negative (~3-5% of biopsy-confirmed celiac patients are seronegative)

Classic Biopsy Findings (Marsh Classification)

• Villous atrophy (partial to subtotal)
• Crypt hyperplasia
• Increased intraepithelial lymphocytes (>25 per 100 enterocytes)

Why the Other Options Are Wrong

• IgA anti-tTG - Best screening test, NOT confirmatory alone; can be falsely positive
• IgA anti-endomysial antibody - High specificity but still serological; cannot replace biopsy
• IgG anti-tTG - Used only when IgA deficiency is suspected; not the confirmatory standard

Yamada's Textbook of Gastroenterology: "A false-positive diagnosis for a disorder that requires life-long, sometimes burdensome, treatment is not acceptable. For this reason, findings characteristic of celiac disease on duodenal biopsy remain the gold standard for celiac disease diagnosis."

Harriet Lane Handbook: "Biopsy is 'gold standard' for diagnosis. Intestinal biopsies showing villous atrophy supports diagnosis."

Creasy & Resnik's Maternal-Fetal Medicine: "The gold standard of diagnosis remains biopsy of the small intestine... from the distal duodenum."

Answer: B - Tight Junctions Between Capillaries

The Full Mechanism (Guyton & Hall)

"The pulmonary capillaries and pulmonary lymphatic system normally maintain a slight negative pressure in the interstitial spaces; whenever extra fluid appears in the alveoli, it is sucked mechanically into the lung interstitium through the small openings between the alveolar epithelial cells. The excess fluid is then carried away through the pulmonary lymphatics. Thus, under normal conditions, the alveoli are kept 'dry.'"

The Three-Component System:

Why the Other Options Are Wrong

Clinical Correlation - When Tight Junctions Break Down

Guyton and Hall Textbook of Medical Physiology: "This filtration pressure causes a slight continual flow of fluid from the pulmonary capillaries into the interstitial spaces... this fluid is pumped back to the circulation through the pulmonary lymphatic system."

Answer: D - All of the Above

Detailed Breakdown

1. Sympathetic Nervous System Control (Option A - TRUE)

• β2-adrenergic receptors on bronchial smooth muscle mediate bronchodilation when stimulated by epinephrine (circulating) or sympathetic norepinephrine
• This is the dominant physiological mechanism for bronchodilation - which is why β2-agonists (salbutamol, terbutaline) are the mainstay of asthma treatment
• During stress/exercise, sympathetic activation widens bronchioles to increase airflow

Note: While "mainly under sympathetic control" is stated as Option A, the word "mainly" is debatable - resting bronchomotor tone is actually predominantly parasympathetic (vagal). However, the sympathetic system is the more powerful dilatory influence.

2. Muscarinic (Parasympathetic) Control (Option B - TRUE)

• The vagus nerve (CN X) provides parasympathetic innervation to the bronchi and bronchioles
• Postganglionic fibers release acetylcholine (ACh) which acts on M3 muscarinic receptors on smooth muscle
• M3 stimulation causes bronchoconstriction
• This is why muscarinic antagonists (ipratropium, tiotropium) are used as bronchodilators in COPD
• Resting bronchomotor tone is largely maintained by tonic vagal (muscarinic) activity

3. Local Stimuli - Narrowing (Option C - TRUE)

Summary Table