Furuncle or Boil: A furuncle is an infection of the hair follicle that can cause a painful, hard nodule or pimple, often with a central core or "ball" which is actually a collection of pus and dead tissue. This condition is common and fits the description provided.

Cystic Acne: This is a type of acne that can cause large, painful pimples with hard centers due to the accumulation of sebum, dead skin cells, and bacteria. The hard ball inside could be a result of the body's attempt to encapsulate the infection.
Epidermoid Cyst: Also known as sebaceous cysts, these are benign cysts that can appear as hard, ball-like structures under the skin. They are usually filled with a cheesy, white material and can become inflamed, resembling a pimple.
Ingrown Hair: An ingrown hair can cause a small, hard bump, especially if it becomes infected. The "hard ball" could be the ingrown hair itself or a collection of pus.

Methicillin-Resistant Staphylococcus aureus (MRSA) Infection: MRSA can cause skin infections that may resemble a pimple with a hard center. It's crucial to consider this diagnosis because MRSA infections can be severe and require specific antibiotic treatment.
Squamous Cell Carcinoma: Although rare, a long-standing, hard nodule could potentially be a sign of skin cancer, such as squamous cell carcinoma. Early detection is key for effective treatment.

Keratoacanthoma: A rare, low-grade malignancy that can appear as a firm, round nodule with a central crater, which might be mistaken for a pimple with a hard ball inside.
Dermatofibroma: A benign skin growth that can appear as a hard, small bump, often resulting from minor injury, which might fit the description given.
Pilomatrixoma: A rare, benign tumor of the skin that can present as a hard, mobile nodule, which could be confused with a pimple containing a hard ball.