A 57 year-old patient presents the following symptoms: for several months he had noticed weakness, sore tongue, acroparesthesias and diarrhea. Examination reveals pallor, absence of position and vibration sensation in the feet, and as atrophic tongue, blood counts shows a microcytic anemia. The one of the following which will cause the best response of reticulocytosis is:

Correct Answer: B

Rationale: Microcytic anemia (MCV <80 fL) with weakness, glossitis, neuropathy suggests iron deficiency ferrous sulfate (B 2 g daily) boosts Hb, reticulocytes (e.g., 5-10% in 7 days) by restoring iron (e.g., ferritin <15 μg/L). Folate (A) treats macrocytosis. Thiamin (C) is B1, unrelated. Transfusion (D) is temporary. None' denies. Iron's efficacy is key, guiding nursing for GI tolerance and anemia correction.

Correct Answer: C

Rationale: Dead space refers to the portion of the tidal volume that does not participate in gas exchange. Anatomical dead space includes the conducting airways (e.g., trachea, bronchi), while physiological dead space includes both anatomical dead space and any alveolar dead space (alveoli that are ventilated but not perfused). The statement that physiological dead space is the same as alveolar dead space is false because physiological dead space encompasses both anatomical and alveolar components, not just the latter. Measuring physiological dead space involves the Bohr method, which uses mixed expired PCO2, arterial PCO2, and tidal volume, so that statement is true. Mechanical ventilation can increase dead space by adding apparatus dead space (e.g., tubing), and an increased ventilation/perfusion (V/Q) ratio can occur in conditions like pulmonary embolism, where ventilation exceeds perfusion, both of which are accurate. The false statement hinges on the incorrect equivalence of physiological and alveolar dead space, as physiological dead space is a broader concept that includes all non-gas-exchanging volumes, not limited to poorly perfused alveoli.

Correct Answer: B

Rationale: Airway resistance is inversely proportional to the fourth power of the radius (Poiseuille's law), but total cross-sectional area also determines resistance across the respiratory tree. The trachea has a large diameter (~2 cm), but as a single tube, its cross-sectional area is limited (e.g., ~3-4 cm²). Bronchioles, though individually small (~1 mm), number in the thousands by the terminal stage, yet their collective area is still less than the alveoli. The alveoli, numbering ~300 million in adult lungs, have a tiny individual diameter (~0.2 mm) but an enormous total cross-sectional area (~70-100 m² during inspiration), vastly exceeding other structures. This massive area reduces airflow velocity and resistance to negligible levels at the alveolar level, where gas exchange occurs by diffusion, not flow. While resistance is highest in medium-sized bronchi due to turbulent flow, the alveoli's collective area minimizes overall resistance to air movement, making them the site of lowest resistance, contrasting with the trachea or bronchioles, which handle bulk airflow with higher resistance despite larger individual diameters.

Correct Answer: B

Rationale: The percentage of CO2 in a structure reflects its PCO2, tied to metabolic production and gas exchange. Pulmonary arteries carry deoxygenated blood from the right heart to the lungs, with a PCO2 of ~45-46 mmHg (venous blood), the highest among options, as it's loaded with CO2 from systemic tissues. Alveolar air has a PCO2 of ~40 mmHg, equilibrated with arterial blood after CO2 diffuses out during respiration. Pulmonary veins, post-gas exchange, carry oxygenated blood with a PCO2 of ~40 mmHg, matching arterial levels. Interstitial fluid's PCO2 varies but approximates venous blood (~45 mmHg) or slightly less, depending on local metabolism, though it's not a standard respiratory measure. Systemic arteries, not listed, also have ~40 mmHg. Pulmonary arteries stand out with the highest CO2 due to their role in transporting metabolically produced CO2 to the lungs for excretion, before alveolar ventilation lowers it, making them the site of peak CO2 concentration.

Correct Answer: C

Rationale: Functional residual capacity (FRC) is the lung volume after a normal expiration (~2.5-3 L), the resting state where lung inward recoil balances chest wall outward recoil. It's the lung's resting volume, but also reflects the thorax's state, though these aren't mutually exclusive options. The key true statement is that at FRC, intra-alveolar pressure equals atmospheric pressure (~760 mmHg), as no airflow occurs (P = 0 gradient), and muscles are relaxed. Intrapleural pressure (IPP) at FRC is negative (~-4 mmHg, 756 mmHg), not more than atmospheric (760 mmHg), due to recoil forces keeping lungs expanded rising above atmospheric only in pathology (e.g., pneumothorax). Lung compliance varies with volume, not lowest at FRC, which is a mid-range point. The equality of alveolar and atmospheric pressure at FRC is a fundamental respiratory principle, ensuring stability at rest, making it the standout true statement.