Other than the fact that the disease needs to be fatal, I'd say a disease like carnitine palmitoyltransferase II (CPT2) deficiency would fit your requirements. I've sourced a lot of my information from here, but I've also taken some information from a recently streaming show on Netflix called Diagnosis, specifically the first episode.
CPT2 is essentially a genetic disorder that keeps your body from processing fats. There are three forms of this disease, two of which are fatal or near-fatal and show up while the carrier is still an infant. The third form, the myopathic form, is the least severe and usually doesn't present symptoms until adolescence. This form would almost perfectly fit with your ill character.
"This form is characterized by recurrent episodes of muscle pain (myalgia) and weakness and is associated with the breakdown of muscle tissue (rhabdomyolysis). The destruction of muscle tissue releases a protein called myoglobin, which is processed by the kidneys and released in the urine (myoglobinuria). Myoglobin causes the urine to be red or brown. This protein can also damage the kidneys, in some cases leading to life-threatening kidney failure. Episodes of myalgia and rhabdomyolysis may be triggered by exercise, stress, exposure to extreme temperatures, infections, or fasting. The first episode usually occurs during childhood or adolescence. Most people with the myopathic form of CPT II deficiency have no signs or symptoms of the disorder between episodes." (Genetics Home Reference).
In many real-world cases of this disease, the myopathic form has been shown to have a devastating impact on the carrier but once diagnosed, it is relatively easy to manage. Since their bodies can no longer process fats, people with CPT2 will typically supplement their energy intake with lots and lots of sugar and protein. In a normal person, sugar, or glucose, is the primary source of energy, while fats stored throughout the body are a secondary source only used when the body has run out of its quickly metabolized source of sugar. Since CPT2 patients' fat stores are completely useless when it comes to metabolization, energy must be supplemented from elsewhere. Many of those diagnosed with CPT2 may find it beneficial to keep several low-fat candy bars on hand at all times for when they begin to feel the onset of their excruciatingly painful symptoms.
Now for an in-depth look at the symptoms of carnitine palmitoyltransferase II (CPT2) deficiency:
- Myalgia: This symptom is the severe and agonizing muscle pain that results from the patient's body breaking down muscle tissue to use as an alternate energy source since its fat stores are now utterly useless for the same task. This breakdown of muscle is called rhabdomyolysis.
- Myoglobinuria: This next symptom is a result of the myoglobin found in muscles being processed by the kidneys. Myoglobin, in this case, could be described as microscopic needles tearing through the bloodstream of the unfortunate individual: horribly painful. Upon reaching the kidneys, the myoglobin enacts similar havoc on the filter organs until it is eventually processed out and excreted through the urine. The myoglobin in the urine results in the patient's pee looking somewhat like coffee grounds upon exit.
- Kidney failure: Just as described above, myoglobin is not meant to be processed en masse by the kidneys. The normal amount of myoglobin in the bloodstream is less than 90mcg/l (micrograms per liter). Most people with CPT2 have around 5000mcg/l and in some severe cases can have upwards of 30,000. If left untreated, kidney failure is inevitable.