Medicare Parts A, B, C, and D Explained: The Plain-English Guide for Adult Children

Medicare has four parts, labeled A through D, and they were apparently named by someone who wanted to make healthcare as confusing as possible. Each part covers different things. Some are mandatory, some are optional. Some are free, some cost money. And the way they interact with each other is not intuitive at all.

If you're helping a parent navigate Medicare for the first time, this is the foundational knowledge you need before making any decisions. Everything else -- choosing between Medigap and Medicare Advantage, picking a drug plan, understanding enrollment periods -- builds on understanding what these four parts actually do.

Medicare Part A: hospital insurance

Part A covers inpatient care. Think of it as the "big stuff" that happens inside a hospital.

What Part A covers

• Hospital stays. Room, meals, nursing care, medications administered during the stay, lab tests, and surgeries performed as an inpatient
• Skilled nursing facility care. Up to 100 days per benefit period, but only after a qualifying hospital stay of at least 3 consecutive days (this rule catches many families off guard -- read about the observation status trap that can disqualify your parent)
• Home health care. Part-time skilled nursing or therapy services ordered by a doctor
• Hospice care. Comfort care for terminal illness, including medications for pain management

What Part A does NOT cover

• Long-term care (assisted living, custodial nursing home care)
• Most dental, vision, and hearing services
• Care outside the United States (with very limited exceptions)

What Part A costs

Most people pay $0 in premiums for Part A because they (or their spouse) paid Medicare taxes for at least 40 quarters (10 years) during their working life. If your parent did not work long enough to qualify, the premium can be up to $518 per month in 2026.

Even with $0 premiums, Part A has significant out-of-pocket costs:

• Hospital deductible: $1,676 per benefit period (2026). This resets each benefit period, not each calendar year
• Days 1-60: $0 coinsurance after the deductible
• Days 61-90: $419 per day coinsurance
• Days 91+: $838 per day (using lifetime reserve days, of which there are only 60 total)

These costs are exactly why most financial advisors recommend supplemental coverage -- either Medigap or Medicare Advantage -- on top of Original Medicare.

Medicare Part B: medical insurance

Part B covers outpatient care. Think of it as everything medical that happens outside a hospital admission.

What Part B covers

• Doctor visits and specialist appointments
• Outpatient surgeries and procedures
• Diagnostic tests (X-rays, MRIs, blood work)
• Durable medical equipment (wheelchairs, walkers, oxygen)
• Mental health services
• Preventive care (annual wellness visits, cancer screenings, flu shots)
• Ambulance services
• Some home health services

What Part B does NOT cover

• Most prescription drugs (that's Part D)
• Routine dental, vision, and hearing care
• Cosmetic surgery
• Long-term care
• Care outside the United States

What Part B costs

Unlike Part A, everyone pays a monthly premium for Part B. The standard premium in 2026 is $185.00 per month. However, if your parent's income exceeds certain thresholds, they pay more through a surcharge called IRMAA (Income-Related Monthly Adjustment Amount).

After paying the premium, Part B has:

• Annual deductible: $257 (2026)
• Coinsurance: 20% of Medicare-approved amounts after the deductible, with no out-of-pocket maximum

That last point is critical and often overlooked. Original Medicare Part B has no cap on out-of-pocket spending. If your parent has a major health event -- cancer treatment, cardiac surgery, extended physical therapy -- they owe 20% of every bill with no ceiling. A $200,000 surgery means $40,000 in coinsurance. This is the single biggest reason people buy supplemental coverage.

Medicare Part C: Medicare Advantage

Part C is not a separate benefit. It is an alternative way to receive Parts A and B (and usually Part D) through a private insurance company instead of directly through the federal government.

Medicare Advantage plans are offered by companies like UnitedHealthcare, Humana, Aetna, and Blue Cross Blue Shield. The government pays these companies a fixed amount per enrollee, and the company manages benefits, networks, and costs.

What Part C typically includes

• Everything Part A covers
• Everything Part B covers
• Usually Part D drug coverage (called MA-PD plans)
• Often extra benefits not included in Original Medicare: dental, vision, hearing, gym memberships, transportation to appointments

The tradeoffs

Medicare Advantage sounds like a better deal on paper -- more benefits, often lower premiums. But there are significant tradeoffs that matter enormously when your parent actually gets sick:

• Network restrictions. Most Advantage plans are HMOs or PPOs. Your parent must use in-network doctors and hospitals, or pay much more (or get nothing covered at all with an HMO)
• Prior authorization. The plan can require approval before covering certain treatments, specialists, or medications. Denials are common -- the HHS Office of Inspector General found that Medicare Advantage plans denied prior authorization requests at concerning rates
• Geographic limitations. Many Advantage plans only work in specific counties or states. If your parent travels or lives part of the year in another state, coverage may not follow them

For a deeper look at the tradeoffs, read the full Medigap vs. Medicare Advantage comparison.

What Part C costs

• Monthly premium: Many plans charge $0 beyond the Part B premium your parent already pays. Some charge $20-$100+
• Out-of-pocket maximum: By law, Advantage plans must cap annual out-of-pocket costs. In 2026, the maximum allowed is $8,850 for in-network services. Many plans set their caps lower
• Copays and coinsurance: Vary by plan and service type

Medicare Part D: prescription drug coverage

Part D covers outpatient prescription drugs -- the medications your parent picks up at a pharmacy. It is offered through private insurance companies, either as a standalone plan (paired with Original Medicare) or built into a Medicare Advantage plan.

How Part D works

Part D plans have their own premium, deductible, and cost-sharing structure. The most important concept is the formulary -- the list of drugs each plan covers. Different plans cover different medications and assign them to different cost tiers, which directly affects what your parent pays at the pharmacy counter.

Key 2026 changes

The Inflation Reduction Act made major improvements to Part D:

• $2,000 annual out-of-pocket cap. Once your parent has paid $2,000 in out-of-pocket drug costs, they pay nothing more for covered prescriptions for the rest of the year
• $35 monthly insulin cap. No Medicare beneficiary should pay more than $35 per month for insulin
• Medicare Prescription Payment Plan. Your parent can opt to spread out-of-pocket costs into equal monthly payments instead of paying large amounts upfront

For the full breakdown, read our guide to the Medicare donut hole changes in 2026.

What Part D costs

• Monthly premium: Varies by plan, typically $10-$80+ per month
• Annual deductible: Up to $590 in 2026 (some plans have no deductible)
• Late enrollment penalty: If your parent does not enroll in Part D when first eligible and does not have other creditable drug coverage, they face a lifetime penalty of 1% of the national base premium for every month they delayed

How the parts fit together: the two paths

Understanding the four parts is important, but the real decision comes down to how your parent combines them. There are essentially two paths:

Path 1: Original Medicare + Medigap + Part D

• Part A (hospital) + Part B (medical) = Original Medicare
• Add a Medigap policy to cover the gaps (deductibles, coinsurance, that unlimited 20% Part B exposure)
• Add a standalone Part D plan for prescriptions

Best for: Parents who want to see any doctor who accepts Medicare (no network), who travel frequently, or who want predictable costs with minimal surprise bills.

Path 2: Medicare Advantage (Part C)

• One plan replaces Parts A, B, and usually D
• Often includes dental, vision, and hearing extras
• Lower premiums but with network restrictions and prior authorization requirements

Best for: Parents who are generally healthy, who live in one area year-round, who are comfortable with an HMO or PPO structure, and who want the lowest monthly premium.

Neither path is universally better. The right choice depends on your parent's health, medications, doctors, travel patterns, and financial situation. If you're trying to help them decide, the Medigap vs. Medicare Advantage comparison walks through the decision in detail.

The gaps none of these parts fill

Regardless of which path your parent chooses, there are services Medicare simply does not cover:

• Long-term custodial care (nursing homes, in-home aides for daily living activities)
• Routine dental (cleanings, fillings, dentures -- some Advantage plans offer limited coverage)
• Routine vision (eye exams for glasses, frames, lenses)
• Hearing aids (some Advantage plans include partial coverage)
• Care outside the US (Medigap Plans C through N offer limited emergency foreign travel coverage)

These gaps are significant and worth planning for separately, especially long-term care, which can cost $8,000+ per month.

Understanding Parts A through D is the first step. The next step is making smart decisions about how to combine them, when to enroll, and which plans to avoid. If you want a structured walkthrough of the entire process -- from enrollment timelines to plan comparison worksheets -- the Medicare Enrollment Guide organizes everything into a printable action plan for $14.