Functional unit of the kidney.

## The Reasoning The **nephron** is the functional unit of the kidney — meaning it's the smallest structure that can perform all the kidney's essential jobs: filtering blood, removing waste, and regulating water and salt balance. Think of it like this: if the kidney is a factory, the nephron is the individual worker on the assembly line doing the actual work. Each kidney contains about **1 million nephrons**, and each one has parts like the glomerulus (filters blood) and tubules (reabsorb useful substances, secrete waste). The principle here: **Every organ has a "functional unit"** — the basic working part that carries out its main function. --- ## Why the Wrong Options Tempt You - **Neuron** — Functional unit of the **nervous system** (brain, spinal cord). Students mix up similar-sounding words. - **Alveolus** — Functional unit of the **lungs** (where gas exchange happens). - **Villus** — Functional unit of the **small intestine** (absorbs digested food). All are functional units, just for *different organs*. The trick is matching the right unit to the right organ. --- ## Quick Takeaway **Nephron = Kidney's worker; it filters your blood and makes urine.**