A person breathes into and from a spirometer (volume 12 liters) containing 10% helium gas mixture. After equilibration, helium concentration of expired gas was found to be 6.67%. His ERV is 4.2 liters. What is his residual volume? (Hint: V1C1=V2C2)

Correct Answer: C

Rationale: Residual volume (RV) is calculated via helium dilution, where helium doesn't enter blood, diluting based on lung volume. Spirometer volume (V1) = 12 L, initial helium concentration (C1) = 10% (0.1), final concentration (C2) = 6.67% (0.0667). Per V1C1 = V2C2: 12 × 0.1 = V2 × 0.0667, 1.2 = V2 × 0.0667, V2 = 1.2 / 0.0667 ≈ 18 L. V2 is total volume (spirometer + FRC). FRC = V2 - V1 = 18 - 12 = 6 L. FRC = ERV + RV, with ERV = 4.2 L, so RV = 6 - 4.2 = 1.8 L = 1800 ml. Equilibration at FRC (post-normal expiration) is standard. The 1800 ml reflects helium's dilution by unexpired lung air, aligning with RV's role as the non-exhalable volume, matching physiological norms.

Correct Answer: C

Rationale: Convection is the primary heat transfer mechanism from Earth's surface to the atmosphere. Solar radiation heats the surface (~168 W/m² absorbed), warming air via conduction (~24 W/m²), but convection dominates as warm air rises, transferring heat vertically (~97 W/m² latent + sensible heat, per energy budgets like Trenberth). Radiation (~396 W/m² emitted, ~333 W/m² back via greenhouse) is surface-to-space, not directly to atmosphere. Conduction is minor due to air's low conductivity. Advection moves heat horizontally, not vertically primary. Convection's buoyancy-driven circulation (e.g., thunderstorms) outpaces other modes, making it the key process, per climate models, for atmospheric heating.

Correct Answer: A

Rationale: The pterygopalatine fossa is a key junction. The maxillary artery enters via the pterygomaxillary fissure (A) from the infratemporal fossa, branching to supply the nose, palate, and pharynx. The maxillary nerve (V2) enters from the middle cranial fossa (C) through the foramen rotundum, exiting variably (e.g., infraorbital canal, D, for the face). At the pterygomaxillary fissure (A), the artery enters while the nerve is already within, heading elsewhere, fulfilling different directions.' B (infratemporal fossa) is the artery's origin, not passage point; C and D are partial paths, not the junction. A is the precise anatomical site of divergence.

Correct Answer: D

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.

Correct Answer: A

Rationale: Paramesonephric (Müllerian) ducts, in females, develop into the uterus and upper vagina (A), forming under estrogen influence as the mesonephric ducts regress. Ovaries (B) arise from gonadal ridges (mesoderm), not paramesonephric ducts, which are separate from gamete-forming tissues. Testes (C) and prostate (D) develop in males from mesonephric (Wolffian) ducts under testosterone, while paramesonephric ducts degenerate. A is correct paramesonephric ducts' role in female reproductive tract formation contrasts with the others' distinct embryological pathways.