Valves prevent backflow in specific veins only

Steps:

• Identify vessel types: capillaries exchange fluids, arteries carry blood from heart, renal vein drains kidneys.
• Recall valve function: valves are one-way flaps to stop blood reversing in low-pressure systems.
• Check each: capillaries lack valves due to thin walls; arteries have high pressure, no need; renal vein is a large abdominal vein without valves.
• Match to options: only all "x" (absent) fits.

Why B is correct:

• Veins contain valves only in extremities to counter gravity; renal vein, as a major visceral vein, has none per circulatory anatomy.

Why the others are wrong:

• A: Wrongly places valve in capillary, which has no muscle or need for backflow prevention.
• C: Incorrectly adds valve to renal artery, which relies on elastic walls and pressure, not valves.
• D: Erroneously includes valves in both renal artery and vein, ignoring abdominal vein structure.

Final answer: B