Parsimonious. My teachers taught me this principle at the bedside during my residency. A single diagnosis should explain all symptoms and signs whenever possible.

That was in 1980. Physicians relied on their brains and stethoscopes. They took detailed histories, examined patients thoroughly, ordered few tests, discussed differentials, and took pride in getting the diagnosis right.

I didn’t know then that my teachers were referring to Occam’s razor—a logical principle often applied in medicine. It states that the simplest, single explanation is usually correct. When patients presented with fever, a heart murmur, and an enlarged spleen, we diagnosed infective endocarditis. When alcohol-dependent patients arrived with swollen legs, yellow eyes, and a distended abdomen, we attributed everything to liver cirrhosis.

We rarely considered that a febrile patient with a heart murmur could also have typhoid. Or that a long-term smoker might die from a pulmonary embolism rather than an exacerbation of COPD. Or that an anxious girl gasping in the ER might have diabetic ketoacidosis rather than a psychosomatic disorder.

Yesterday, a medical student reminded me of an important lesson: Never close a differential diagnosis too soon.

For three weeks, she had a blocked, runny nose. ENT specialists diagnosed acute bacterial sinusitis. Low-grade fever, headaches, and yellow nasal discharge fit the picture. She took antibiotics, antihistamines, and decongestants. Her fever subsided—then returned. She felt drained, self-diagnosed allergies, and prepared for her pre-final exams, exhausted and anxious.

Her father, a physician, travelled 20 hours by train to see her. He asked if I could visit her in the guest house where she had temporarily moved from her hostel. I agreed.

She looked weak. I asked pointed questions, traced the illness’s timeline, and noted her fever patterns. I checked her vitals, sinuses, throat, and chest. Her nasal lining was red, her throat congested. “My nose is still blocked,” she said, “and my headaches are terrible.” She blamed allergies. I reassured her. “The infection may be resistant to the antibiotic,” I told her father. I prescribed a new one, hoping it would work.

Her father remained silent. I sensed his unease. He wanted a full workup. I asked her to visit me in the OPD.

The next morning, she came in. I ordered blood tests, malaria screening, a Widal test, and X-rays of her chest and sinuses. I also sent her for another ENT consultation.

Three hours later, her father called. Plasmodium falciparum. Life-threatening malaria.

Relief. Shame. Relief that the lab provided an answer. Shame that it hadn’t come from my own reasoning. I had fixated on sinusitis, distracted by her talk of allergies. I hadn’t considered malaria—despite practicing in Sevagram, where we see a hundred severe malaria cases each season.

Why had I missed it? I had closed my differential diagnosis too early. Occam’s razor had misled me. I assumed one cause explained everything. I hadn’t asked, “What else could this be?”

In How Doctors Think, Jerome Groopman describes why physicians miss diagnoses. Not from ignorance, but from cognitive traps. He offers a simple three-question checklist:

  1. What else could this be?
  2. Could two conditions be present at once?
  3. Is anything inconsistent with my working diagnosis?

Had I asked those questions, malaria would have surfaced sooner.

How do we avoid these errors? Approach each case with fresh eyes. Keep an open mind. Don’t rush to closure. And without ordering unnecessary tests, remember that a simple checklist can prevent a serious mistake.